details:
- setting format options from python isnt possible anymore since this isnt exposed via op->properties, python should use image.save() function instead.
- image save UI now hides 'Relative' option when copy is selected since it has no effect.
- default image depth is set to 8 or more if the image has no float buffer, otherwise its set to 32 or less.
other fixes:
- image new was adding an image with a filepath set to "untitled", if this file happened to exist in the current directory a save on the generated image would overwrite it, now initialize to empty path.
- BKE_ftype_to_imtype was returning an invalid value if ftype==0.
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