util/blitter: add clamping during SINT <-> UINT blits

util/blitter: add clamping during SINT <-> UINT blits

Even though glBlitFramebuffer cannot be used for SINT <-> UINT blits, we
still need to handle this type of blit here because it can happen as part
of texture uploads / downloads, e.g. uploading a GL_RGBA8I texture from
GL_UNSIGNED_INT data.

Fixes parts of GL45-CTS.gtf32.GL3Tests.packed_pixels.packed_pixels.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>