Ignore devicePixelRatio if it's undefined.

In older versions of Firefox, such as version 17, this causes the display to fail.
Adjust logic to be a bit clearer, and add comments.
Fix SVG path regression with FullscreenButton.

Simply subscribing to a computed observable does not force the computed observable to be evaluated.  If it's never otherwise evaluated, the subscription will never fire.  To work around this, extract the subscribeAndEvaluate helper function from within Animation, and use it everywhere we add a subscriber, to ensure correct behavior no matter what by forcing the evaluation to occur.
Merge pull request #1269 from AnalyticalGraphicsInc/handleUndefinedDevicePixelRatio

Ignore devicePixelRatio if it's undefined.
Merge pull request #1270 from AnalyticalGraphicsInc/fixFullscreenButtonPath

Fix SVG path regression with FullscreenButton.
Merge remote-tracking branch 'origin/master' into gltf