meta: Only clear the requested color buffers.

This path is used to implement both glClear and glClearBuffer; the
latter is only supposed to clear particular buffers.  Core Mesa provides
us that information in the buffers bitmask; we must only clear buffers
mentioned there.

To accomplish this, we save/restore the color draw buffers state, and
use glDrawBuffers to restrict drawing to the relevant buffers.

Fixes Piglit's spec/!OpenGL 3.0/clearbuffer-mixed-formats and
spec/ARB_framebuffer_object/fbo-drawbuffers-none glClearBuffer tests
for drivers using meta clears (such as Broadwell).

Cc: "10.2" <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77852
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77856
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Anuj Phogat <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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