This implements system level DPI awareness for Windows. It has three
parts:
1. Add a application manifest which is copied alongside servo.exe during
build that declares our DPI awareness level. This is needed otherwise
DPI queries will return 96dpi and our application will be upscaled on
high DPI displays.
2. Rename hidpi_factor to avoid confusion with Glutin's hidpi_factor
which does something else.
3. Correctly convert windows sizes on window creation for
Windows. Unlike OS X, Windows uses device pixels for window creation.
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