meta: Add support for glClear() to integer color buffers.

meta: Add support for glClear() to integer color buffers.

This requires using a new fragment shader to get the integer color
output, and a new vertex shader because #version has to match between
the two.

v2: Clarify that there's no need for BindFragDataLocation.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)