Don't crash if there are too many vertices in a row + softgpu frameskip

Might not be realistic, but we crash if we go over.  Pretty unlikely to happen in real games, but I suppose not impossible.  Happens in the vertex speed demo, so fixes #3106.

The demo still shows only black, might be pixel poking?  I'm not sure.  Anyway, it does work in the softgpu.

Softgpu really needed frameskip too.  Seems to work okay.

-[Unknown]