Allow changing the size of coordinate system

[qgustavor]

One of the features of svg2ssa is allowing changing the size of coordinate system to improve precision. This feature is documented here changes \p1 to \p# where # is a integer.

Checking the current code scaling mode is fixed at \p1. From my experiments seems implementing such a feature is just a matter of adding a setting for it, changing the line linked above to return \p# and scaling values as needed (positions ~~and stroke widths~~).