[MP] Rend2: Increase BSP max VBO size to 16MB

On very large maps, 4MB was creating too many vertex buffers. 16MB reduces
the number of VBOs to a reasonable number (< 10) without over-shooting for
too much memory on smaller maps.
[MP] Rend2: Port new ioq3 MD3 tangent calculations

This fixes some code in the MD3 tangent calculations which looked a bit
dodgy, and nobody really understood what it was trying to do.
[MP] Rend2: Change G2 tangent calcs to match MD3