meta: Don't botch color masks when changing drawbuffers.

Color clears should respect each drawbuffer's color mask state.

Previously, we tried to leave the color mask untouched.  However,
_mesa_meta_drawbuffers_from_bitfield() ended up rebinding all the
color drawbuffers in a different order, so we ended up pairing
drawbuffers with the wrong color mask state.

The new _mesa_meta_drawbuffers_and_colormask() function does the
same job as the old _mesa_meta_drawbuffers_from_bitfield(), but
also rearranges the color mask state to match the new drawbuffer
configuration.

This code was largely ripped off from Gallium's st_Clear code.

This fixes ES31-CTS.draw_buffers_indexed.color_masks, which binds
up to 8 drawbuffers, sets color masks for each, and then calls
glClearBufferfv to clear each buffer individually.  ClearBuffer
causes us to rebind only one drawbuffer, at which point we used
ctx->Color.ColorMask[0] (draw buffer 0's state) for everything.

We could probably delete _mesa_meta_drawbuffers_from_bitfield(),
but I'd rather not think about the i965 fast clear code.  Topi is
rewriting a bunch of that soon anyway, so let's delete it then.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94847
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>

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