Suggestion to improve shader and graphics design

[AntonPetrov83]

Using a lot of if ... else in the fragment shader is suboptimal plus XOR, NXOR drawing modes are not supported.
I want to try and address this. The current plan I have is:

1. Implement shader for different drawing modes using preprocessor definitions to avoid condition checks in the fragment shader.
2. NOR and XOR modes will be using the current canvas as a uniform texture source.
3. Render in pure black and white colors (zeroes and ones) to simplify logic and blending math.
4. Implement final output shader which will map black and white image from the previous step to nice color values or perform some post processing like CRT effect etc.

[AntonPetrov83]

At the moment I have proof-of-concept implementation for XOR and NXOR drawing modes but it requires changing canvases to make frambuffer available in the shader every drawing call (!!!) I think this will be completely inappropriate in some cases.

Moving towards the normal PR...