Split up the SvgRenderer monolith

[adroitwhiz]

Expected Behavior

SVG renderer functionality should be exported in a granular way, so each place that depends on it can take only what it needs (e.g. 2.0 -> 3.0 SVG conversion, measuring SVGs, deserializing SVGs, actually drawing SVGs). This is blocking scratchfoundation/scratch-render#594.

Actual Behavior

Currently there's lots of functionality mixed into SvgRenderer, some of which is used in scratch-render, some of which is used in scratch-vm, etc. This should all be separated out, especially since #127 will be adding lots of scratch-render-specific SVG rendering code, and it would be cleaner for that to be able to live in scratch-render.

[fsih]

This looks like a really good improvement! It could also be a really helpful step towards some of our other SVG issues. I'm going to talk to our tech lead about it to make sure it's ok, and then we'll start looking at putting this in.