i965/blorp: Implement source clipping.

i965/blorp: Implement source clipping.

This patch modifies blorp blits (which are used for MSAA) to properly
account for clipping of source coordinates.  Previously, if we
detected the possibility of source clipping, we would fall back to the
blit meta-op, which doesn't support MSAA and is very slow for depth
and stencil buffers.

Fixes piglit tests
"EXT_framebuffer_multisample/clip-and-scissor-blit" on i965/Gen6+.

Also substantially speeds up the Humble Bundle V game "Psychonauts" on
Gen6+ (without this patch, the game's depth buffer blits use the slow
blit meta-op).

Reviewed-by: Carl Worth <cworth@cworth.org>