mesa: add statevar for sample position palette

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz
glsl: add builtin uniform for sample positions palette

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
glsl: add interpolateAtSample builtin

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>