meta: Add pixel store/pack operations in decompress_texture_image

meta: Add pixel store/pack operations in decompress_texture_image

This patch adds the pixel store operations in decompress_texture_image().
decompress_texture_image() is used in glGetTexImage() for compressed
textures with unsigned, normalized values.

It also fixes the failures in intel oglconform pxstore-gettex due to
following sub test cases:

 - Test all mipmaps with byte swapping enabled
 - Test all small mipmaps with all allowable alignment values
 - Test subimage packing for all mipmap levels

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=40864

Note: This is a candidate for stable branches

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>