nine/swapchain: set PIPE_BIND_SAMPLER_VIEW on back and DS buffers
nine/pipe: don't vary rasterizer.point_size if point_size_per_vertex
nine/state: D3DRS_CLIPPING only affects software vertex processing
nine/state: only SM < 3 vertex colors have to be clamped

And fixed-point PS outputs are supposed to be clamped independently
of the rasterizer setting.

R600g doesn't support clamp controls so just keep it simple.