Bug 1097293: Do not draw radial gradients when centers and radii are identical. r=jrmuizel

When Cinner and Couter are equal, as well as Rinner and Router, the algorithm described in the canvas specification will hit a division by 0. As described in section 7 of the Canvas2D specification nothing should be drawn in this situation.
Bug 1097476: Fix typo in D2D 1.1 radial gradient code. r=jrmuizel
Bug 1097628: Mark tests as fuzzy with D2D even when Canvas & Content backends are identical. r=jwatt
Bug 1082827: Allow D2D 1.1 to be selected for the canvas backend. r=bas
Bug 1083664 - Disable test_dataChannel_basicAudio.html on OSX debug for frequent assertions.

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extra : rebase_source : 253249f776e464781588eb62a7b3373faf44243e