fix type errors in GraphicContext

[bcolloran]

fix type errors in GraphicContext. Untangles that mess a bit by:

• cleaning up cascadingContextStore. It's only called in one place, so it doesn't need generic params and a static argument can be pushed into the function
• making the the default overrides more explicit rather than spreading DEFAULT and pruneProps(...) objects on top of each other, which leads to ambiguous types.