Resolve multiple inconsistencies when 3D text could not load fonts.
- When the font wasn't found:
The default font was used.
- When the font existed but could not be loaded:
The 3D text would fail to display anything.
- When only the VFont assosiated with a style failed to load:
The 3D text would fail to display only when the stype existed.
So toggling bold for example could cause all 3D text to vanish.
Instead of falling back to defaults, display a placeholder glyph
(a hollow rectangle) displayed when the font can't be used.
For the UI fallbacks are fine, for user content, it indicates something
is wrong as needs to be corrected - similar to a bright magenta in
renders when images fail to load. This means it's clear to users when
they have broken paths which need to be corrected.
This also simplifies vfont_to_curve, removing a "goto" to go to the
end of the function to cancel 3D text rendering entirely.
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