wlr_scene: Be resilient against overflow conditions

If the area calculations for output overlap overflow a signed int, we
may not consider it to be a primary output. Turn this into an unsigned
type so this happens less frequently.
Additionally, it is possible the overflow would produce 0, we can handle
this by simply changing the comparison to more than or equal.

While we're here, let's assert that we always assign a primary output
if there are any intersecting outputs.

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