main: Simplifies a bit _mesa_unpack_color_span_ubyte() func from pack.c

Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>
main: Simplifies a bit _mesa_unpack_color_span_float() func from pack.c

Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>
main: Removes _mesa_unpack_bitmap() in favor of the more generic func _mesa_unpack_image()

Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>
Merge pull request #1 from Igalia/elima-format-pack-glgetteximage

elima's commits from work on pack.c