Some more GDI cleanup stuff...

Create a proper aspect fix function
Centre bitmap in window with correct aspect ratio
Fix cursor at resize points
Explicitly set high DPI awareness
Don't snap window larger than allowed

Since this is the newest/simplest display driver, I'm going to use
this to decide how to do scaling in general so I can abstract that
into shared code to ensure all drivers have the same scaling
behaviour.

I haven't tested mouse on windows with black bars yet though, so I
assume that's broken...