- glReadPixels() was running to get the depth on each pixel, this works fine one some cards but was locking up on OSX.
- Replace glReadPixels() call with a single call to view3d_update_depths() right after view3d_validate_backbuf(), so the depths are only read once.
- Unrelated to the changes above, but should improve performance: view3d_validate_backbuf() was being called on every redraw while combing, now only call once when combing starts.
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