On high DPI displays, a logical pixel does not necessarily equal a
device pixel. Images and such are scaled up to the logical pixel size;
unfortunately this introduces ugly aliasing. Web pages can take
advantage of high DPI screens by intentionally scaling stuff down to be
displayed at a higher resolution. By carefully canceling out the
automatic scaling of the game canvas we get nice crisp pixels even on
high DPI screens.
See the pull request for before/after screenshots.
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