Support conditional render

This series adds support for NV_conditional_render and it passes 10 out of 12 piglit related to the extension on WARP.

It fails nv_conditional_render-generatemipmap because of some unrelated error with "resizing windows too often" Manually run the piglits passes, and it fails nv_conditional_render-copypixels because it seems that with glCopyPixels the y-flip is not handled correctly. Changing the piglit to do the copying as horizontal subsections instead of vertical ones makes it pass.

The piglits don't tests queries that were suspended and resumed again, so that the accumulation is not really tested, and the accumulation is still done on the CPU.

Further work would be to use a compute shader to accumulate the result.