This fixes bugs related to how an `Image` follows the size of an SVG.
We track the "source size" of each image, i.e. the original width/height
of the SVG, which can be different from whatever it was rasterized as.
* Closes https://github.com/emilk/egui/issues/3501
The problem occurs when you want to render the same SVG at different
scales, either at the same time in different parts of your UI, or at two
different times (e.g. the DPI changes).
The solution is to use the `SizeHint` as part of the key.
However, when you have an SVG in a resizable container, that can lead to
hundreds of versions of the same SVG. So new eviction code is added to
handle this case.
