surface: redesign state

• Rename invalid to committed - better ideas welcome!
• Move subsurface_position in wlr_subsurface
• Fix sx/sy handling
• Don't monkey-patch surface_damage and buffer_damage - add a new buffer damage region instead
• Make current and pending embedded structs
• Add a previous state
• Update cursor hotspot with buffer position

Test plan:

• Make sure damage tracking works on surfaces and subsurfaces (try gnome-calculator)
• Try with scaled and transformed outputs
• Try wleird's cursor example

See #1065 (closed)