Background class, and 5 examples.

[DragonMoffon]

The push Implements a background class, and BackgroundTexture, BackgroundGroup, and ParallaxGroup. Plus five examples: static Background, scrolling Background, BackgroundGroup, ParallaxGroup, and Blending between Backgrounds.

These close #1152 and #991

[einarf]

Some quick intial comments. This actually looks really solid, but I am not 100% sure about the details around what properties should be on the texture and the background itself to give the most flexbility.

Also there are missing some type annotations here, but I think fixing the more obvious things and just merging it is a better approach here. Play around with the feature and tweak things instead.

Adding a RGBA color multiplier can also add a lot of flexiblity here instead of just blend. We can really merge this as it is now to be honest. Knitpicking on smaller things that are subject to change around a little bit won't move us forward.

[DragonMoffon]

For the colour should, do you think I should apply via multiply or addition? Currently, have it as multiply, but if someone wanted to fade in the color, it would be easier if it was added.

Also, I have it supporting both normalised and not, but I decide if the colour is normalised by seeing if the sum of the colour is less than or equal to 3, which may cause some confusing bugs. Is there a better way of deciding?

[einarf]

• We do color multiplication everywhere in arcade right now, so that would make the most sense
• If you want to support byte RGBA and normalized RGBA color they can have different properties. color, color_normalized

[DragonMoffon]

Some of the functions are still missing docstrings. Which will need to be fixed before 2.7 is out. But otherwise, it should be mergeable.