gapit screenshot -overdraw didn't generate output image

**Environment information:**
- GAPID version: 1.6.1
- Host OS: Windows 10 x64
If tracing on Android:
- Device model: Google Pixel 4 XL
- Android Version: 10

**Bug description**
`gapit screenshot -overdraw a.gfxtrace` didn't generate a output image

**Reproduction steps**
Run `gapit screenshot -overdraw a.gfxtrace` on a single frame capture.
Expected result: a screenshot.png saved to local drive that contains overdraw count instead of actual pixel color.
Actual result: no output file generated at all.

Have verfied that `gapid screenshot a.gfxtrace` work all fine.

**Stacktrace**
Please paste a stacktrace here if it's available.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Additional debugging information**
[logcat.txt](https://github.com/google/gapid/files/4513556/logcat.txt)
[hostlog.txt](https://github.com/google/gapid/files/4513580/hostlog.txt)