Optimize editor

[RealKC]

The editor currently does a lot more work than needs to be done when receiving draw events, here are some things that could be done to improve the performance:

• Avoid some allocations in editor::pixelops by avoiding the intermediate Pixbufs
• Pull the editor::pixelops code into its own crate and config cargo to enable all optimizations on that one (this is most important for people running kcshot in debug mode -- which I am)
• Figure out a way to cache previously drawn surfaces, so we don't need to execute all operations over and over each draw event

I'll update the issue if more things come to mind.