Merged requested to merge gerddie/virglrenderer:fix-gs-clipdist into master
If we always emit the glip distance in GS we might end up having too many outputs, which will result in failures.
Hence, for GS only emit the clip distance evaluation code when the clip planes are enabled.
Fixes: 072f3095 shader: Always write code to toggle clip plane
Closes: #254 (closed)