i965/msaa: Implement glSampleCoverage.

i965/msaa: Implement glSampleCoverage.

This patch enables glSampleCoverage() functionality, which allows the
client program to specify that only a portion of the samples be lit up
when performing multisampled rendering.  i965 supports
glSampleCoverage() through the 3DSTATE_SAMPLE_MASK command packet,
which allows the driver to specify a bitfield indicating which samples
to light up.

Fixes piglit tests "EXT_framebuffer_multisample/sample-coverage {2,4}
{inverted,non-inverted}".

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>