[duim,gtk] get-gcontext: Assert on null window.

This leads to subsequent errors and shouldn't happen, so let's
assert here.
[duim,gtk] Enable more code in update-drawing-state.

This enables calls to establish-pen and establish-font. It also
comments out the dash-handling code in establish-pen as it is
buggy. It then makes the ignoring messages more specific and
correct.