mag_rate incorrectly being applied in full screen mode

[rakslice]

Testing on Linux on a 4k display (3840x2160), with mag_rate set to 2, I found it would sometimes affect full screen mode, effectively taking up the whole screen with the top left corner of the emulated Mac display.

Situations to test:

• Initial display setting full screen
• Toggling to full screen with Ctrl + Enter
• Changing resolution
• Changing colour depth