Update image "x/ydata" when "cdata" is changed (bug #42121) * graphics.in.h (image::properties): add two hidden properties "xdatamode" and "ydatamode" * graphics.in.h (image::properties): modify properties "xdata" and "ydata" so that their respective *mode is updated * graphics.in.h (image::properties::update_cdata): setting "cdata" updates "x/ydata" when "x/ydatamode" is "auto" * graphics.in.h (image::properties::update_x/ydata): setting "x/ydata" to empty matrix updates "x/ydata" to [1 npix] and changes "x/ydatamode" to "auto" * graphics.in.h (image::properties::auto_xdata (void), image::properties::auto_ydata (void)): new functions to compute auto x/ydata * image.m (__img__): don't replace empty "x/ydata" in order to use new behavior * image.m: add test for bug #42121

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