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.

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ET maps do not have BSP flares, so I assume this wasn't causing problems. I found it using ET dlight culling code with Q3 maps. https://github.com/zturtleman/spearmint/commit/7c0a62a8e4004df241dc5171b70644572cbfa5cb