Coordinate space translation

[Chris3606]

The goal of this PR is to implement a system that can satisfy Chris3606/GoRogue#281. To that end, it makes the following changes:

• SadRogue.Primitives.UnboundedGridViews namespace has been added
  • Contains interfaces similar to IGridView (and ISettableGridView), except they don't have the constraints of coordinates must start at 0, 0 and have a defined width/height. Basically, these are grid views that can represent an infinite map that uses an arbitrary coordinate space
• All viewport-related grid views have been moved to their own namespace (SadRogue.Primitives.GridViews.Viewports)
  • Generic IVewport interface now acts as a base interface for all viewport-related implementations
  • The old UnboundedViewport is now called DefaultValueViewport
  • Viewports (UnboundedViewport and UnboundedSettableViewport now exist that expose a sub-area of an IUnboundedGridView as an IGridView; this represents a camera (as an IGridView) of an arbitrary coordinate space
• Added SadRogue.Primitives.CoordinateSpaceTranslation namespace
  • Provides ways of defining translations between two coordinate spaces
  • Provides an extension method for IViewport which generates a coordinate space translator appropriate for a viewport
  • Provides a grid-view like interface for exposing a grid view of local coordinates as global coordinates