libgimpbase: return 0 for GIMP_UNIT_PIXEL in gimp_unit_get_factor().
Just as documented, pixel unit should always return factor 0. There is no need to call _gimp_unit_vtable.unit_get_factor(). This is even more important as there is one implementation of unit_get_factor() in core, and another in libgimp and the one in libgimp is expecting unit to always be >= GIMP_UNIT_INCH. So we were getting CRITICALs in libgimp when calling gimp_unit_get_factor() on pixel unit (for instance when drawing a GimpRuler).
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