Blender Git Loki

GPUSelect: Don't read depth on every draw

If the draw uses the same id as the previous draw, there is no need to read the depth
buffer at this point, avoiding a CPU-GPU sync bubble.

Fixes T62511 Selection is significantly slower in production scenes.
With this patch glReadPixels is not the bottleneck. Regular drawing Is still very
slow so I would suggest fixing the regular drawing first before trying to
improve the selection algorithm.