GLUI menu driver doesn't support alpha channel images

First and foremost consider this:

• Only RetroArch bugs should be filed here. Not core bugs or game bugs
• This is not a forum or a help section, this is strictly developer oriented

When developing the PR https://github.com/libretro/RetroArch/pull/17216, it was noticed when testing the glui menu driver does not support alpha channel images. See here.

Expected behavior

glui menu driver supports alpha channel images

Actual behavior

glui menu driver does not support alpha channel images

Steps to reproduce the bug

1. Place alpha channel image in place of a Boxart or Snapshot or Title image
2. Change to the glui driver
3. Note it shows black background instead of alpha channel

Bisect Results

I think it's always happened, but never noticed with Boxart, Snapshot or Titles. If logos are introduced as the PR suggests, it will be noticeable.

Version/Commit

You can find this information under Information/System Information

• RetroArch: 1.19.1

Environment information

• OS: Winx64
• Compiler: libretro-build-mxe-win64-cross

Start by reproducing the issue with an alpha-channel image used as Boxart, Snapshot, or Title imagery while the GLUI menu driver is active. Inspect the GLUI menu driver's image rendering path and compare its behavior with the PR-linked changes; done means transparent regions display correctly instead of with a black background.