fix: ColorMapFilter alpha issues

[albinkong]

I have ran into some issues with the ColorMapFilter and alpha. I created a
reproduction (zip) that can be run locally and have provided screenshots of the demo below.

In general the filter did not properly handle the premultiplied nature of the textures and had some issues with still having the blue channel as linear when set to nearest.

I've also updated the documentation for the color map texture as well as added an example color map to the filter's folder.

There are two breaking changes in this PR: the keys in ColorMapFilter.uniform have changed, and the way the filter looks will be less "glitchy" (most obvious on the bottom image in the "Passthrough 4x4x4, nearest"-example).

Screenshots of different color maps that can be seen in the example code.

Passthrough 4x4x4, linear

Passthrough 4x4x4, nearest

Hue rotate 4x4x4, linear

Hue rotate 4x4x4, nearest

Passthrough 32x32x32, linear

Passthrough 32x32x32, nearest

[albinkong]

I am tempted to add passing FilterWithShader to the constructor, but want to wait for feedback before making such a change.