Utility function to resolve the size of a raster grob
Determine the width and height of a raster grob when one or both are not given explicitly.
The result depends on both the aspect ratio of the raster image and the aspect ratio of the physical drawing context, so the result is only valid for the drawing context in which this function is called.
A raster grob
A raster grob can be specified with width and/or height of
NULL, which means that the size at which the raster is
drawn will be decided at drawing time.
A raster grob, with explicit width and height.
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