Use Gmprat for Path bounds with TRANSFORM_BEZIERS_TO_PATH

So, the bounds of Paths are stored with Gmprat
The Bezier's are all stored relative to the Path, as floats

The transformations are only applied to the Path Gmprat bounds.

This seems to work rather well.

In other news, the DeCasteljau algorithm in the CPU
renderer had no upper limit, which is why it was slowing down so much.

The CPU renderer tends to suffer from SIGFPE-itis when using floats,
because when you do a cast there is a SIGFPE if the resultant type can't represent the operand.
This occurs in a few places that don't actually affect the rendering...