Implement real-time SSAO on the CPU

[Dawoodoz]

Screen Space Ambient Occlusion implemented on a CPU can take advantage of many techniques that are not available in GPU pixel shaders to make it fast.

Dirty rectangles can allow only calculating a filter for the regions that have changed.

Box filtering on a CPU can be done in less runtime complexity than a separable pixel shader on the GPU. Repeated box filtering approximates gaussian blur and can probably be added to the same pass using more registers to save cache.

Other effects such as bloom can also reduce run-time complexity by allowing each following pixel to re-use the result of the previous neighbor. If the GPU is ten times as fast, the CPU will just have to do 10% of the calculations to reach the same results in the same amount of time.