[WIP] Implement surface-layers protocol

• Implement surface-layers in wlroots
• Implement surface-layers in rootston
  • Render at correct position
  • Input types (pointer)
  • Exclusive types: pointer
  • Exclusive types: keyboard
  • Render and send inputs at correct z-depth
  • Exclusive zones N/A
• Build a test client
  • Basic version
  • CLI flags to control everything
• Protocol changes
  • Negative margins
  • Errors
  • Double-buffered state
  • Fix non-exclusive pointer events
  • Fix non-exclusive keyboard events
  • Add a configure event
• Bug fixes
  • Invalid margin: -100,100
  • More than 2 anchors sends a configure request
• Push back protocol changes to sway-protocols

Protocol spec: https://github.com/swaywm/sway-protocols#surface-layers