Add orthographic camera support

[adamkewley]

Estimate: 1-3d

It isn't strictly necessary for model creation, but should be fairly easy to jack into the existing code with some sort of "isOrtho" flag (maybe).

The current scene cameras always use perspective projections with some fixed field-of-view. However, model builders will likely also need orthographic projection support for (e.g.) assigning things with background image help etc.

[adamkewley]

De-prioritized because it's important but there's potentially a new camera abstraction coming along with the 3D backend rewrite.

[adamkewley]

Note: because the 3D backend is going to take time, it might be that a quick isOrtho bool hack is enough to patch it into place while I flesh out the full cameras API with the new backend

[adamkewley]

Estimate revised up a lot because the latest rendering/hittesting etc. code might currently assume that the camera is a frustrum.