empty gif after ffmpeg error "Number of frames to loop is not set"

Original issue was made by @TLINDEN in https://github.com/charmbracelet/freeze/issues/168 and it was moved to VHS.

**Describe the bug**
I have made a rather long tape file which worked fine up to a certain point. But I am unable to find the section of the file which causes the issue. If I comment out all sections, only let one section uncommented and then execute vhs, then it works every time. However, if all sections are enabled, then it fails. So I think the tape file has no errors.

When the issue happens, an empty gif will be created and ffmeg shows an error message.

**To Reproduce**
Steps to reproduce the behavior:
1. Clone https://github.com/tlinden/anydb/
2. `git checkout feature/vhs-demo`
3. `go build`
4. `cd demo`
5. `make` (this actually executes `vhs`)

**Expected behavior**
A gif should have been created.

**Screenshots**
```default
[..]
Sleep 4s

Creating demo.gif...
ffmpeg version 7.0.2-3ubuntu1 Copyright (c) 2000-2024 the FFmpeg developers
built with gcc 14 (Ubuntu 14.2.0-2ubuntu1)
configuration: --prefix=/usr --extra-version=3ubuntu1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --disable-libmfx --disable-omx --enable-gnutls --enable-libaom --enable-libass --enable-libbs2b --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libglslang --enable-libgme --enable-libgsm --enable-libharfbuzz --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzimg --enable-openal --enable-opencl --enable-opengl --disable-sndio --enable-libvpl --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-ladspa --enable-libbluray --enable-libcaca --enable-libdvdnav --enable-libdvdread --enable-libjack --enable-libpulse --enable-librabbitmq --enable-librist --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libx264 --enable-libzmq --enable-libzvbi --enable-lv2 --enable-sdl2 --enable-libplacebo --enable-librav1e --enable-pocketsphinx --enable-librsvg --enable-libjxl --enable-shared
libavutil 59. 8.100 / 59. 8.100
libavcodec 61. 3.100 / 61. 3.100
libavformat 61. 1.100 / 61. 1.100
libavdevice 61. 1.100 / 61. 1.100
libavfilter 10. 1.100 / 10. 1.100
libswscale 8. 1.100 / 8. 1.100
libswresample 5. 1.100 / 5. 1.100
libpostproc 58. 1.100 / 58. 1.100
[Parsed_loop_6 @ 0x6058b1b78c40] Number of frames to loop is not set!
[Parsed_loop_8 @ 0x6058b1b79240] Number of frames to loop is not set!
Input #0, image2, from '/tmp/vhs3282649779/frame-text-%05d.png':
Duration: 00:07:01.12, start: 0.000000, bitrate: N/A
Stream #0:0: Video: png, rgba(pc, gbr/bt709/iec61966-2-1), 1044x837, 25 fps, 25 tbr, 25 tbn
Input #1, image2, from '/tmp/vhs3282649779/frame-cursor-%05d.png':
Duration: 00:07:01.12, start: 0.000000, bitrate: N/A
Stream #1:0: Video: png, rgba(pc, gbr/bt709/iec61966-2-1), 1044x837, 25 fps, 25 tbr, 25 tbn
Input #2, lavfi, from 'color=#171717:s=1200x1000':
Duration: N/A, start: 0.000000, bitrate: N/A
Stream #2:0: Video: wrapped_avframe, yuv420p, 1200x1000 [SAR 1:1 DAR 6:5], 25 fps, 25 tbr, 25 tbn
Input #3, png_pipe, from '/tmp/vhs3282649779/bar.png':
Duration: N/A, bitrate: N/A
Stream #3:0: Video: png, rgb24(pc, gbr/unknown/unknown), 1200x1000, 25 fps, 25 tbr, 25 tbn
Input #4, png_pipe, from '/tmp/vhs3282649779/mask.png':
Duration: N/A, bitrate: N/A
Stream #4:0: Video: png, gray(pc, gbr/unknown/unknown), 1200x1000, 25 fps, 25 tbr, 25 tbn
Stream mapping:
Stream #0:0 (png) -> overlay
Stream #1:0 (png) -> overlay
Stream #2:0 (wrapped_avframe) -> scale:default
Stream #3:0 (png) -> loop:default
Stream #4:0 (png) -> loop:default
paletteuse:default -> Stream #0:0 (gif)
Press [q] to stop, [?] for help
[Parsed_loop_6 @ 0x706834019100] Number of frames to loop is not set!
[Parsed_loop_8 @ 0x7068340197c0] Number of frames to loop is not set!

Host your GIF on vhs.charm.sh: vhs publish .gif
```
**Desktop (please complete the following information):**
- OS: Kubuntu 24.10
- Browser: chromium
- `vhs version v0.9.0 (a5f09b2)`

**Additional context**
Many thanks for this awesome program, I love it! :)

## Commentary

Ok, I have extracted the precise ffmpeg commandline executed by vhs:

```shell
ffmpeg -y -r 50 -start_number 1 -i /tmp/vhs3763011057/frame-text-%05d.png -r 50 \
-start_number 1 -i /tmp/vhs3763011057/frame-cursor-%05d.png -f lavfi -i \
color=#171717:s=1200x1000 -i /tmp/vhs3763011057/bar.png -i /tmp/vhs3763011057/mask.png \
-filter_complex "[0][1]overlay[merged]; [merged]scale=1080:850:force_original_aspect_ratio=1[scaled];
[scaled]fps=50,setpts=PTS/1.000000[speed];
[speed]pad=1200:970:(ow-iw)/2:(oh-ih)/2:#29283b[padded];
[padded]fillborders=left=60:right=60:top=60:bottom=60:mode=fixed:color=#29283b[padded] ;
[3]loop=-1[loopbar]; [loopbar][padded]overlay=0:30[withbar] ;
[4]loop=-1[loopmask]; [withbar][loopmask]alphamerge[rounded] ;
[2]scale=1200:1000[bg]; [bg][rounded]overlay=(W-w)/2:(H-h)/2:shortest=1[withbg] ;
[withbg]split[plt_a][plt_b]; [plt_a]palettegen=max_colors=256[plt];
[plt_b][plt]paletteuse[palette]" -map "[palette]" demo.gif
```

I also saved the dir `/tmp/vhs3763011057/` containing the png frame images away. Then I run the ffmpeg command myself which leads to the same error as above.

I also looked if the provided frame and cursor images matched - they did, that is, no image was missing. There were 14514 text and 14514 cursor frames.