Feature/high resolution screenshot

[seankmartin]

Feature goal:
Support taking cropped screenshots from neuroglancer of only the data panels. Support taking these at x1, x2, or x4 scale.
In x2 and x4 scale, the canvas is resized. This means that 2D panels kind of "zoom out" to support the extra pixels. 3D panels on the other hand keep their view fixed, but have more pixels in the framebuffer.

Overall idea of files/classes:

1. The python integration ScreenshotHandler is what actually interacts with the data to try and grab screenshot data, and repeatedly grab statistics while this is loading.
2. The neuroglancer ScreenshotManager interacts with the display context to start the ScreenshotHandler process. It also receives information from the ScreenshotHandler and passes this on to the screenshot UI menu when appropriate. It saves the file when complete, and stops screenshots automatically if they hang.
3. The ScreenshotDialog handles the user input to trigger actions in the ScreenshotManager, and shows updates from the ScreenshotManager to the user.