Use GL_RGB565 internal format when available.

This is a feature from ES2_compat extension.
It fixes the speed issue associated with using 16-bit textures on
desktop GL. Improves performance a bit as well as there's less bandwidth
usage during shading.

On my HD3000 laptop, performance improved with ~10%.
Comment about BGRA8888.
Merge branch 'per-core-config' of github.com:libretro/RetroArch into per-core-config
Merge branch 'per-core-config' of github.com:libretro/RetroArch into per-core-config