gallium: add bits for clipping points as tris (d3d-style)

gallium: add bits for clipping points as tris (d3d-style)

OpenGL does whole-point clipping, that is a large point is either fully
clipped or fully unclipped (the latter means it may extend beyond the
viewport as long as the center is inside the viewport). d3d9 (d3d10 has
no large points) however requires points to be clipped after they are
expanded to a rectangle. (Note some IHVs are known to ignore GL rules at
least with some hw/drivers.)
Hence add a rasterizer bit indicating which way points should be clipped
(some drivers probably will always ignore this), and add the draw interaction
this requires. Drivers wanting to support this and using draw must support
large points on their own as draw doesn't implement vp clipping on the
expanded points (it potentially could but the complexity doesn't seem
warranted), and the driver needs to do viewport scissoring on such points.

Conflicts:

	src/gallium/drivers/llvmpipe/lp_context.c
	src/gallium/drivers/llvmpipe/lp_state_derived.c

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>