Revamp camera and viewport handling

[mkeeter]

• Adds View2 and View3 types to the Fidget crate itself, which define world-to-model transform matrices. Remove the existing Camera type.
• Adds a RegionSize<N> (typedefed to ImageSize and VoxelSize) for a pixel size, which gives the screen-to-world transform matrix
• Uses these types in RenderConfig (2D / 3D images) and Settings (for meshing)

Todo:

• Use these types in demos
• Remove the separate camera utility library?
• Update CHANGELOG