The `Canvas2dMsg::MeasureText` is dropped inside `send_canvas_2d_msg` if
the canvas is not paintable,
leading to a panic on the receiving end. Checking the paint-ability
before sending the message prevents this panic, and if the canvas is not
pain-table, a default text metrics is used.
Testing: Manual testing of the minimal test case in the associated
issue, and crash test added.
Fixes: https://github.com/servo/servo/issues/36845
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Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>