Basic support for High DPI with 2x scale factor (ie Retina).
This can be enabled by setting 'highdpi': 2 in the simpleWindow settings.
A different approach is taken than the Cocoa point/pixel translation.
For Plask it makes more sense to always work in pixels, and basically the
highdpi flag just controls the window size and weather the backing store is
scaled up or not.  Since the events will still be done in Cocoa points, they
are scaled back to pixels in the Plask event handling code.
Note, this doesn't set NSHighResolutionCapable in the plist.