Tile grid 3D physics support

**Is your feature request related to a problem? Please describe (REQUIRED):**
Yes, I'd like to see use raycast to check if enemies exist above or below a tile grid. I would like to use accurate 3D distance figures in my game for range calculation.

**Describe the solution you'd like (REQUIRED):**
Treat tile grid as a plane. If a raycast collision occurs, return the type of the tile (set in the tilesource) as the group.

**Describe alternatives you've considered (REQUIRED):**
Working around the issue by using the on-screen x/y position of enemies with different Z-values but using the correct Z-value when releasing a projectile at them.

**Additional context (OPTIONAL):**
Error shown when 3D physics is used with tile grid: "ERROR:GAMESYS: Collision objects in 3D can not have a tile grid as shape."