Regression in OpenGL converter

Commit 05f29287 fixed a chroma issue with vaapi and OpenGL ES2, but introduced a regression in vlc-ios#1299 (closed).

However, the former version seemed correct when the format is using GL_LUMINANCE_ALPHA, which has (L,L,L,A) swizzle, like in https://code.videolan.org/videolan/vlc/-/blob/aeedea5abf9f4031964bf95dee5034bc2219c501/modules/video_output/opengl/interop.c#L185.

ALPHA is used there to provide the second value from the two-component plane, and should be sampled as a, or previously w.

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