Add resolutionScaleFactor to CesiumWidget and Viewer.

This isn't ready for master because I need some help, but it's mostly done.  It adds a new property to CesiumWidget to scale the rendered resolution of the scene, so 0.5 makes a 640x480 widget render the scene at 320x240, 2.0 makes it render at 1280x960, etc.. It might be useful if you are targeting low-powered graphics hardware hooked to high-res screens (like phones, tablets, or even desktops with retina displays).  I could see some apps setting the scaling factor dynamically based on system performance (@kring aren't you working on some sort of widget related to that?)

One issue that I noticed is that billboard/label behavior seems to be the opposite of what I expect.  Lower resolution makes bigger billboards.  I'm not sure how easy/hard that would be to fix (and if it's hard it might render this entire PR moot).  Anyone have any thoughts on this?  This is easy to play with using any Sandcastle example.