Allow GPU generator to start on next view while waiting for previous one.

[Kneelawk]

Currently the GPU-based fractal generator will submit a command buffer to render a fractal and then wait for it to complete before submitting the next command buffer. It would be nice if the GPU-based generator would submit a command buffer and immediately start working on the next one while waiting for the previous one to complete.

[Kneelawk]

There is one large issue with this though, that is that currently everything only uses one set of textures and buffers. In order for this to work properly, there would need to be multiple sets of textures and buffers, allowing the generator to work on one while the GPU was rendering to the other.