WIP: lavapipe: add support for conservative rasterization.

Marked WIP due to misrendering, which might suggest the approach is subtly wrong somewhere.

This attempts to add basic support for conservative raster.

It's a bit hacky, it "uses" the multisample renderer to and moves the sample points to the 4 corners, when rendering it takes the coverage masks for all 4 samples and ORs them together and uses the result to render.

This works except for the bottom corners of the tri with the vulkan conservative rendering sample.