gallium: add basic support for ARB_sample_shading
mesa/st: add support for ARB_sample_shading
nv50: add support for PIPE_CAP_SAMPLE_SHADING
nvc0: add support for PIPE_CAP_SAMPLE_SHADING

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
mesa/st: translate gl_SampleMaskIn to SAMPLEMASK semantic

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nvc0/ir: add support for SAMPLEMASK sysval

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>