mesa/format_pack: Add _mesa_pack_int_rgba_row()

mesa/format_pack: Add _mesa_pack_int_rgba_row()

This will be used to unify code in pack.c.

v2:
- Modify pack_int_*() function generator to use c.datatype() and
  f.datatype()

v3:
- Only autogenerate pack_int_*() functions for non-normalized integer
  formats.

v4:
- Use _mesa_unsigned_to_unsigned() in pack_int_*() because, in order
  to be able to pack both signed and unsigned formats, we need to
  sign-extend.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>