fix(developer): crash exporting OSK on European keyboard

Fixes #5609.

If a user attempted to export the OSK to a BMP or PNG, and their base
keyboard was a European layout (102 key), then Keyman Developer would
crash as it attempted to recalculate and redraw the keyboard when
constructing the OSK object, and there would be no window for the
OSK object to draw to (or measure dimensions against).

This fix uses a temporary off-screen canvas to facilitate calculation of
dimensions, and avoids drawing or invalidating if no window is
available.