FIX mesa: get_tex_rgba_uncompressed() allow MSVC compilation

MSVC is not fully C99 compliant and in this case, declarations cannot
follow code.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
FIX mesa: fix out-of-tree build error in autogenerating pack/unpack format functions

NOTE: Need to figure out a better way of doing this.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>