Cap the number of vertexes per flush.

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 (#3106.)
Add GPU cycle estimation to the softgpu.
Support frameskip in softgpu.
Merge pull request #4343 from unknownbrackets/gpu-minor

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