Move duplicate code from ParseFace() and ParseTriSurf() into ParseTriangleSurface()

Also moves `plane` from `srfSurfaceFace_t` to `srfSurfaceGeneric_t`, since it's the only difference between them, and plane computations were already being done on `srfSurfaceGeneric_t` anyway, so they can be moved there too.

Author: VReaperV