This allows the viewport to be scaled to compensate for DPI scaling on
high-DPI monitors. Previously, if DPI scaling was set to 200% in Windows, for example,
noVNC would set the client area to twice the dimensions of the server-side framebuffer
and scale it up to fit, leading to the VNC session looking grainy and low-res.
useNativeDpi can now be set to true to compensate for this, and
let framebuffer pixels exactly match monitor pixels.
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