Draft: cursor: overhaul

Depends on !3970

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This MR greatly simplifies wlr_cursor, which at the moment does multiple things at once:

• applies (device-specific) coordinate transformations → this functionality is moved out to a helper
• acts as a proxy for pointer-like devices → removed
  • can be reintroduced as another helper, but that doesn't seem very useful
  • this is the reason why some input events still have the device pointer in them
• manages wlr_output_cursor objects (this includes setting surfaces/images) → the only thing wlr_cursor does now