meta: Clip src/dest rects in BlitFramebuffer, using the scissor

meta: Clip src/dest rects in BlitFramebuffer, using the scissor

Fixes piglit's fbo-blit-stretch test on drivers which use the meta path.
(i965: should fix Broadwell, but also fixes Sandybridge/Ivybridge/Haswell
since this test falls off the blorp path now due to format conversion)

V2: Use scissor instead of just mangling the rects, to avoid texcoord
rounding problems. (Thanks Marek)

V3: Rebase on Eric's CTSI meta changes; re-add _mesa_update_state in the
CTSI path so that _mesa_clip_blit sees the correct bounds.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77414
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Tested-by: Anuj Phogat <anuj.phogat@gmail.com>