3.0: Revert "opengl: fix swizzle"

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)