Add an "arbitrary shape" drawable

[Flutterish]

Resolves #3510 Allows you to draw a vector-graphics-ish shape defined by a closed curve (of vertices). There are 3 fill rules to select from: NonZero (default for vector graphics), EvenOdd and Fan (for convex shapes)

The implementation can also check for "insidedness" of a point in the ReceivePositionalInputAt method. Please let me know if the FillRule docs are a bit confusing - I don't really know how to word them better so we will need to figure something out together.