Fix Issue #1150 plus some small refactoring

Solves Issue #1150 (closed) as well as removes unnecessary Camera FloorZ/CeilingZ functions. MobjFloorZ / MobjCeilingZ functions also sped up by using R_PointInSubsector instead of R_IsPointInSector.

Hopefully this try is a lot cleaner, concise, and easy to code review.