feat: [#2416] Add Postprocessor hooks and default some uniforms

[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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Closes #2416

This PR adds some new default uniforms supplied to postprocessors automatically if they are present in the source

• uniform float u_time_ms - total playback time in milliseconds
• uniform float u_elapsed_ms - the elapsed time from the last frame in milliseconds
• uniform vec2 u_resolution - the resolution of the canvas (in pixels)

Custom uniforms can now be set in an optional onUpdate() function on postprocessors, the internal shader.use() is called for users.

Example using the CRT post processor, see gist https://gist.github.com/eonarheim/5ebb7f9161c10894935479e0caa0f6c7