glx: Iterate over FBConfig and select 8 bit color size
Texture upload mechanism used by gstreamer-vaapi relies on 8 bpc. In latest mesa versions the first fbconfig might not be 8 bit, so iterate over it to find the correct config with supported values.
This is an alternative version of !152 (closed) with below differences:
- Configure GLX_ALPHA_SIZE to 8 bit (required to get this working)
- Iterate using a loop instead of copy-paste same code block multiple times
- Loop breaks if any value is incorrect instead of collecting all values for each FBConfig (should be faster)
- Also check for alpha value which must be either 0 (no alpha) or 8 bit
Fixes: #136 (closed)