gg: always use 4 channels in init_sokol_image

[larpon]

Fixes vlang/vab#239

Will probably obsolete #16561

At some point init_sokol_image() was changed to use the number of channels detected by stbi instead of the hardcoded 4. While this should've been a good idea, and the most logical thing to do - reality is a little different. sokol_gl expects .rgba8/4 channels in it's default contexts, which gg currently also uses when loading images. This PR set the number of channels back where it came from and leaves a more juicy note for next time :)

I don't think it's a good idea to manipulate the channel number like this in unexpected places:
https://github.com/vlang/v/pull/15981/files#diff-3566edbb636a40b3c608bb8b51fb2403f57929e1626e96f39ccb7256f0573a99R121
The correct channel info is nice to have in other situations - the "problem" is the sokol_gl default contexts which expects 4 channels. So I've removed the ugly workaround in this PR as well

I guess this is a good reason to look at getting v gret visual tests up and running for Android/vab as well...

I've tested the PR against the images provided in #16024 with the image viewer also - they all load and work except for basn3p08.png and basn3p08.gif which also both fail to load in GIMP.