Add ability to capture images from USB-connected cameras

When a doctor clicks the "capture" button in the viewer, it saves the current 3D model, plus a PNG of all cameras in the system. Steven has asked if we could add additional high-resolution 2D cameras to the system for captures. This should be fairly straightforward. We should be able to modify the viewer code so that when the capture button is clicked, it also checks for any connected USB cameras, and captures an image from those as well.

This issue is the main issue to track discussion. Please track progress on the below steps in their individual sub-issues.

Step 1 - Investigate
Look for examples online of capturing still images from a camera in Unity. There should be plenty of these. Find something that seems reliable and fast.

Step 2 - Design
Determine where in the viewer code this code should be added. It should only have to be added in the C# code, and not require any changes to the actual Unity project.

Step 3 - Implement
We can compile the Unity code here at MSR if ECL doesn't have a Unity license.

Step 4 - Test
Plug in any available USB camera, doesn't have to be high quality, and test to see if it works. Once we know it works we can discuss where to obtain a camera of the quality Steven wants. We have quite a few here in MSR we can bring down if necessary.

Start by locating the Unity viewer code that handles the capture button and review reliable C# examples for capturing still images from USB cameras. Done means connected USB cameras are detected and their images are saved alongside the existing 3D model and camera PNG captures, then verified with an available USB camera.