Layered drawing is very slow

[Nami Verasche]

Although it leads to tidy code and simplicity in changing things, the layered drawing is very slow. Various improvements could be made to this.

[Nami Verasche]

Commit b471976 contains some significant fixes in this area, but they do result in choppy scrolling.

[Nami Verasche]

Perfomance, even with the layered system, now exceeds that of the existing stable version. As such, while there probably is room for even further improvement, I would consider it good enough to mark this as "resolved" now.