Instead of immediately constructing the bitmap with the requested size,
possibly by downscaling a higher-resolution bitmap, and then potentially
having to upscale it if we actually need a bitmap of a bigger size, just
retrieve the bitmap in the actually needed size from wxArtProvider when
needed.
This makes bitmaps obtained from wxArtProvider::GetBitmapBundle() look
good, rather than fuzzy and ugly, in high DPI if they're actually
available in the appropriate size.
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