The motivation here is to concentrate display options in the GAL display
settings, ready for removal of legacy canvases. Instead of having the
property as a member of the DRAW_FRAME, with the GAL canvas retreiving
it from there, it is now in the GAL_DISPLAY_OPTIONS struct, and both GAL
and legacy get it from there.
The options for setting cursor shape are then moved out of the general
options dialog, and into the GAL display options widget, where they can
be used in all GAL-aware programs.
GAL cursor shape works on GAL, but not legacy, so the option is now
available on OSX (but only affects GAL, and is labelled as such).
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