Improve the blur post-processing shader

[stuarthayhurst]

Replace the current implementation with a new algorithm, with configurable quality (such as sample count)

• Texture sampler settings / quality can also be considered

[stuarthayhurst]

Gaussian seems like a good bet, drop the depth part and just make it a blur shader

[stuarthayhurst]

Probably worthwhile building a post-processing system after the graphics code clean up, use ping-pong framebuffers for Guassian blur.