Update compositor shutdown code

Update compositor shutdown code

Adds a general wlsc_compositor_shutdown() function that all output
backends call when shutting down.  wlsc_compositor_shutdown() will call
a new 'destroy' method of each output to perform backend-specific
cleanup (e.g., turning off the hardware cursor in the DRM compositor).

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>