This adds a decoder for the TinyVG vector format (https://tinyvg.tech/).
TinyVG is a very simple binary vector format, but it is good enough to
represent a lot of SVGs, without needing the full web engine.
The main use case for Serenity is for scalable icons (which .tvg easily
handles).
No real behavior change. (The two globals are now both initialized
at first use instead of before main(), but that should have no
observable effect. The motivation is readability.)