It didn't do anything since wxBitmapBundle-related changes, as could be
seen by toggling the bitmap size in the toolbar sample -- the bitmaps
size didn't actually change.
Make it work again by only using the best bitmap size if it's greater
than the currently set bitmap size, but not shrinking the bitmaps to it
if it's smaller. This doesn't seem especially useful, but this is how
the code behaved with single bitmaps before and there doesn't seem to be
any good reason to change this.
At least document that calling SetToolBitmapSize() is unnecessary and
normally shouldn't be done.
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