Use mozangle instead of NuGet package

[jdm]

Now that the NuGet package is being generated from upstream ANGLE, it would be nice if we could consolidate our ANGLE usage to a single library. Considerations to keep in mind:

• building ANGLE for UWP is documented at https://github.com/servo/servo/wiki/Publishing-a-new-ANGLE-NuGet-version
• nobody's tried to get an equivalent UWP build of mozangle to work
• we'll need to go back to packaging libEGL.dll and libGLESv2.dll manually