Retrieve DPI scale from window instead of screen

[jack-cooper]

Previously, DPI scale was retrieved based on the property from the screen. This PR changes this behaviour to instead retrieve it from the window. Previous behaviour on a macbook could result in a 3:2 ratio between the reported DPI scale and the window size:

WIth high_dpi: false:
screen_size() returns (1440.0, 900.0)
dpi_scale() returns 1.0

With high_dpi: true:
screen_size() returns (2880.0, 1800.0)
dpi_scale() returns 3.0 sometimes and 2.0 somtimes.

The apple docs provide virtually no guarantees for the backingScaleFactor property for NSScreen, while the same documentation for NSWindow explicitly states the returned value is either 2.0 or 1.0. This seems to always line up with the value returned by screen_size, and intuitively I would also expect the value for the window to be of more interest to miniquad than that of the display.

[not-fl3]

Thanks for PR!