Improve geometric picking

With !189 (merged), Mutter has geometric picking available. However, various issues and potential improvements were discovered, and we would like to get it in a better shape for next releases.

• Pick coordinates should be floats
• Investigate and remove the weak ref protection
• Properly split pick and paint routines
• Don't use framebuffer to pass matrices around
• Use the currently under used cache better
• Port to use graphene instead of own baked polygon math functions
• More clean up of the now unused pick color code