weston_surface_assign_output() should prefer primary-backend outputs as a tie-breaker
All else being equal, let's assign a surface to a primary backend's output over a secondary backend's output.
Suggested by @daniels in !578 (comment 1983093).
All else being equal, let's assign a surface to a primary backend's output over a secondary backend's output.
Suggested by @daniels in !578 (comment 1983093).