llvmpipe: fix clearing integer color buffers

llvmpipe: fix clearing integer color buffers

We get int/uint clear color value in this case, and util_pack_color can't
handle these formats at all (even if it could, float input color isn't what
we want).
Pass through the color union appropriately and handle the packing ourselves
(as I couldn't think of a good generic util solution).
This gets piglit fbo_integer_precision_clear and
fbo_integer_readpixels_sint_uint from the ext_texture_integer test group from
segfault to pass (which only leaves fbo-blending from that group not working).

v2: fix up comments