Performance: Reduce the number of PIXI.Graphics objects to a minimum

[flekschas]

The more PIXI.Graphics objects we use the worse our performance gets. So let's reduce the number of graphics objects!

Some easy ones that can be removed:

1. image-with-background.js doesn't need a dedicated graphics object for the background. Let's instead create a 1x1 texture for each background color and use scaled sprites as the background.
2. pile.js might not need contentGraphics at all. We should be able to just replace it with a container.
3. pile.js similar to 1, borderGraphics should be replaced by scaled PIXI.Sprite objects that are instantiated from fixed 1x1 textures. (One texture per color)

Even better would be to replace 1 and 3 with custom meshes. Alternatively, this might also work: https://pixijs.io/pixi-filters/docs/outline_src_OutlineFilter.js.html