renderer: skip unhandled marksurfaces when calculating leaf bounds

BSP flares in leafs cause surfaces after it not to be added to bounds
(surfMins and surfMaxs) used by dlight culling code.

continue is run, but mark++ is not. This causes the flare to be checked
again instead of the the next surface.
Merge pull request #84 from zturtleman/surface_bounds

renderer: skip unhandled marksurfaces when calculating leaf bounds