u_blitter: add integer clear support

u_blitter: add integer clear support

We need add a new set of fragment shader variants, along with new vertex
elements for signed and unsigned clears.

The new fragment shader variants are due to the integers values requiring
CONSTANT interpolation. The new vertex element descriptions are for passing
the clear color as an unsigned or signed integer value.

Signed-off-by: Dave Airlie <airlied@redhat.com>