Investigate freeze during 8x8 4096 imposter bake (general optimization of bake)

[xraxra]

I'm assuming it is Compute Shader related, it might not be dividing up the thread groups in a good way at certain resolutions. Would like to revisit the bake code and better present the baking progress while waiting, since currently there is no indication of progress. This could also help track down what is causing the freeze that was reported.