Fixed P_LookForPlayers scanning redundancies and oversights

- Players could be scanned multiple times, repeating expensive tests
- Players could be skipped completely and become invisible to monsters as a result
- Partial invisibility would cause P_LookForPlayers to end early, causing later players to be checked later

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Fixes this: http://forum.zdoom.org/viewtopic.php?f=2&t=40308