Retain scale when resizing

When calling resize on an enabledElement (maybe because the layout changed or the browser window was resized) there currently seems to be no way to retain the scale of the image. When calling resize with fitViewportToWindow === true the image is resized to fit the viewport (as expected). Calling it with fitViewportToWindow === false then the visible part of the image may be cut off (when the resized canvas is smaller). It would be nice if there is a way to show the image in the same way it was presented before the resize, just larger or smaller.

[medihack]

@dannyrb Yes, @jdnarvaez is right. Especially maximizing/minimizing a view and changing the layout is done very often. And one would expect to retain the correct (or better to say intuitive) scale somehow. I investigated how other DICOM viewers handle this, and it is more complex (and more diverse) than I initially would have thought (funny when I think about how often I use such a viewer at work).

Most viewers behave the following way:

• When the image was never zoomed and never panned then always fit the image to the window size.
• When it was zoomed or panned then do a relative rescale.

I implemented this in the provided pull request and it seems to feel quite natural (at least I hope so). I extracted the function getImageSize in its own file (from fitToWindow) as it is now used in fitToWindow and resize (I had to test somehow in resize if the image fits the window). I also rewrote the resize example to demonstrate the new functionality (just resize the window by dragging the little red square on the right bottom of the window).

I would be happy for suggestions and comments.

[jdnarvaez]

@dannyrb FWIW I see rads/OR users resize viewports a lot when changing layouts and retaining one of the stacks they were displaying previously.