LineTracer interface 3D floors fix

[jewalky]

This is basically a gross hack that makes it use two passes for sectors that have 3D floors, so it retains the original GZDoom execution flow while not requiring raw LineFlags manipulation.
The current behavior is as follows:

1. check for real line
2. call callback
3. if callback returns TRACE_Skip, use recursive call to check for 3D line (but ignore normal lines this time)

This fix should not affect normal engine functioning because this path only activates with TRACE_3DCallback flag (which is currently only set in the ZScript interface).

Resolves #415, but properly.

Test case: http://www.mediafire.com/file/25o3z54y5z9bvml/trace3dfloor_test_zzyzx.wad

P.S. I had other ideas of how to do it, but it's too dangerous and can break other uses of this function.
So for compatibility I just added a slight deviation from normal flow activated by variable.