Investigate potential bug in prior context

[benpious]

The spec specifies that only certain commands should use parts of the "prior context" construct that we use in DrawingCommands.swift to convey information about prior curves in the path.

The fact that we are seemingly more liberal than the spec, and would allow, for example, a T command to use prior context from a C command. This may be responsible for bugs like the one below in rendering the flag of Sri Lanka.