Use the drawing buffer width instead of the canvas width for imagery splitting

[kring]

On systems where the drawing buffer width is not equal to the canvas width in CSS pixels, imagery splitting happens at the wrong position. To see what I mean, visit this URL in Microsoft Edge on a machine with a high DPI display:
http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Imagery%20Layers%20Split.html&label=All

If you can see the split at all, it won't be in the center of the screen as it is on other systems, and it won't line up with the divider line.

Edge:

Chrome:

In case you're wondering what Edge has to do with this: not much. Edge is missing support for the image-rendering CSS style, so Cesium chooses to ignore the devicePixelRatio and render the canvas with full device pixels, which guarantees and CSS pixels and drawing buffer pixels are not the same, triggering the bug. IMO this imagery-rendering behavior is pretty questionable - it would be better for Cesium to be slightly blurry on IE/Edge rather than unusably slow - but that's a separate issue.

[cesium-concierge]

@kring thanks for the pull request!

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[emackey]

Thanks @kring!

[emackey]

BTW this issue can be reproduced on a normal-DPI machine by adding this line to the end of the Sandcastle demo:

viewer.resolutionScale  = 0.3;