* Added an "Override Image" button to the UVProject modifier; this overrides
faces' currently assigned image with the one given in the modifier panel.
This provides some of the functionality previously provided by the "Add UVs"
button - you still need to add a UV layer manually, but you only need to
change the image in one place.
* Note that the "Override Image" option uses the bit position previously used
by the "Add UVs" option, so old files which had "Add UVs" enabled will now
have "Override Image" enabled.
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