Viewport size and position to be defined through percentages or absolute values

[TotalKrill]

Problem

Using viewports to define split screen or minimaps have some differing requirements, that would sometimes be more easily defined by percentage of the available window or absolute values respectively. Minimaps might have more use of absolute values, and split screen would probably have either 50/50 or something similar in viewport.

Solution

This could be expressed as enums for the viewport size and viewports positions as defined by #4898 and proposed by comment

[Weibye]

For inspiration on the implementation side, I know winit is doing something similar when differentiating between Physical and Logical pixel position: https://docs.rs/winit/0.21.0/winit/dpi/enum.Position.html