Add basic screenshot support to 3D viewers

[adamkewley]

Estimate: 2-4d for most of the implementation (incl. UI windows for picking sizes etc.)

Add basic support for screen-shotting the 3D viewer alone.

Of course, users could just do this themselves with print screen/snippet tool. However, the main benefits of having something in-UI is that the UI has direct access to the renderer, so it can:

• Snip out all the UI b.s. around the edges of the render (possible with a snippet tool on most OSes, but is a little annoying to get pixel-perfect with a mouse)
• Disable (optionally) any gizmos in the viewer
• Let the user select a standard resolution + aspect ratio (4:3, 16:9; 300 DPI, etc) for pasting the image into a powerpoint presentation, report, blog post, etc.
• Let the user disable the background elements completely (floor, background color, etc.) so that they can save an alpha-blended image that only contains useful scene elements. Again, this makes it easier to place an image into a presentation slide, which may use a slighly different (maybe, even textured) background.