Post-Processing Effects

[TheCherno]

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This enables a variety of important post effects that can be used to further enhance Sparky's graphics (i.e. screen-space effects, FXAA, colour correction, bloom, etc.). Sparky contains some old code for basic post effects chains, however that will need to be revised.

We need the ability to render to 2D RGB(A) textures first (see issue #105).