Custom internal resolution as in PCSX2

[etking]

In my opinion this change should really be considered to significantly increase Dolphin performance. For 1280x720 Displays, 2x native resolution is a waste of GPU resources and 1280x792 internal resolution would be much better here. And for 1080p, 1920x1056 or 1920x1320 could bring a huge boost in performance without any loss in image quality compared to 1920x1584 since efficiency is key for emulation speed.

Also modern scaling algorithms should be used where possible to maximize image quality and sharpness, especially for the conversion between internal and display resolution.