mesa: Add MESA_FORMAT_SIGNED_RG88 and _RG1616.

mesa: Add MESA_FORMAT_SIGNED_RG88 and _RG1616.

Including pack/unpack and texstore code.  ARB_shader_image_load_store
requires support for the GL_RG8_SNORM and GL_RG16_SNORM formats, which
map to MESA_FORMAT_SIGNED_GR88 and MESA_FORMAT_SIGNED_GR1616 on
little-endian hosts, and MESA_FORMAT_SIGNED_RG88 and
MESA_FORMAT_SIGNED_RG1616 respectively on big-endian hosts -- only the
former were already present, add support for the latter.

Acked-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>