mesa: fix GL_LUMINANCE handling in glGetTexImage

mesa: fix GL_LUMINANCE handling in glGetTexImage

There are several cases in which we need to explicity "rebase" colors
(ex: set G=B=0) when getting GL_LUMINANCE textures:
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

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=46679

Also fixes the new piglit getteximage-luminance test.

Reviewed-by: José Fonseca <jfonseca@vmware.com>