Add a dynamic resolution setting for 3D viewports

[Calinou]

Several modern engines now support dynamic resolution, which is a technique used to achieve more consistent framerates in heavier scenes, especially on slower hardware.
It consists in resizing the rendered viewport depending on the current frame rate; some engines also only render less columns (instead of less columns and less rows) to make the loss of quality harder to notice.

This would be especially welcome for VR, but it could be useful in non-VR scenarios as well.

[Calinou]

Closing in favor of godotengine/godot-proposals#917, as feature proposals are now tracked in the Godot proposals repository.