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

## 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](https://github.com/bevyengine/bevy/pull/4898#issuecomment-1148730290)

Start by reading issue #4898 and the linked comment, which describe the proposed viewport API. Compare that proposal with the current viewport behavior for split screens and minimaps. Done means viewport size and position can be expressed using the proposed percentage-based or absolute-value forms.

Written by the indexing model from the issue text.