GL_LUMINANCE_ALPHA textures should be sampled through r and a swizzle
since it has a (L,L,L,A)
layout.
This reverts commit 4082f4f2, but includes also the fix from c69bbb07 to avoid using an invalid swizzle.
Fixes #26767 (closed) Reintroduce #26735 (closed)
VideoLAN code repository instance