feat: Post Processing Improvements + Pre-multiplied alpha

[eonarheim]

===:clipboard: PR Checklist :clipboard:===

• 📌 issue exists in github for these changes
• 🔬 existing tests still pass
• 🙈 code conforms to the style guide
• 📐 new tests written and passing / old tests updated with new scenario(s)
• 📄 changelog entry added (or not needed)

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Related to v26 roadmap

This PR re-implements post processing using webgl fragment shaders, it is MUCH faster this way. The method of action for this PR uses framebuffers which are encapsulated by ex.RenderTarget which will capture all draw calls and redirect them to a framebuffer texture. This change also requires Excalibur support pre-multiplied alpha because all things end up pre-multiplied in the gpu.

Great articles on premultiplied

• https://www.realtimerendering.com/blog/gpus-prefer-premultiplication/
• https://www.adriancourreges.com/blog/2017/05/09/beware-of-transparent-pixels/

Changes:

• New ex.RenderTarget and ex.RenderSource to encapsulate
• New ex.ScreenRenderer which can draw a RenderSource to the entire screen
• Breaking change to how post processing works, requires fragment shaders
• Slight Breaking change to the ex.ColorBlindnessPostProcessor