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)
  • [x] Set opt-level = 3 for stackblur-iter 6d4b45ada1858222fba6211bad5a549c37fbace3
• [ ] 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.