mesa: Fix GL_LUMINANCE handling for textures in glGetTexImage

mesa: Fix GL_LUMINANCE handling for textures in glGetTexImage

We need to rebase colors (ex: set G=B=0) when getting GL_LUMINANCE
textures in following cases:
1. If the luminance texture is actually stored as rgba
2. If getting a luminance texture, but returning rgba
3. If getting an rgba texture, but returning luminance

A similar fix was pushed by Brian Paul for uncompressed textures
in commit: f5d0ced.
Fixes https://bugs.freedesktop.org/show_bug.cgi?id=47220

Observed no regressions in piglit and ogles2conform due to this fix.
This patch will cause failures in intel oglconform pxconv-gettex,
pxstore-gettex and pxtrans-gettex test cases. The cause of failures
is a bug in test cases. Expected luminance value is calculted
incorrectly in test cases: L = R+G+B.

V2: Set G = 0 when getting a RG texture but returning luminance.

Note: This is a candidate for stable branches.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <idr@freedesktop.org>