vrend: Disable scissors in vrend_clear

vrend_clear executes the Gallium clear command which is only called if the whole viewport is cleared. So far mesa was doing excessive checks on the scissors, thereby updating them also if they were disabled, and the according state changes were transmitted to the host. With mesa/mesa@20374787 this was optimized, so that the host may not disable the scissors as it should before the clear command resulting in an regression in virglrenderer. Disabling the scissors before the clear is executes, and re-enabling them according to the last hw state fixes this. This error only manifests itself with newer mesa.

Closes #116 (closed)

Signed-off-by: Gert Wollny gert.wollny@collabora.com