engines: Add a damage_region_set callback

engines: Add a damage_region_set callback

To properly implement EGL_KHR_partial_update we need to know the buffer
damage before any drawing operations take place.  Add a new callback to
software_generic that takes place after combining of surface damage and
swap mode when we actually have this available.

Note: This means the three copy pasta implementations of
EGL_KHR_partial_update scattered around the tree are all wrong.  bummer.