Cesium broken in latest WebGL2 Chrome implementation

[mramato]

Chrome Canary recently added a WebGL 2.0 Prototype option to chrome://flags and when I tried it out I noticed that Cesium no longer works (if I recall correctly, @bagnell and @lilleyse got it working during our last sprint). Running Cesium Viewer spits out the following errors:

index.html:1 [.CommandBufferContext]GL ERROR :GL_INVALID_OPERATION : glTexImage2D: 
index.html:1 [.CommandBufferContext]GL ERROR :GL_INVALID_FRAMEBUFFER_OPERATION : glClear: framebuffer incomplete
index.html:1 [.CommandBufferContext]GL ERROR :GL_INVALID_FRAMEBUFFER_OPERATION : glDrawElements: framebuffer incomplete
index.html:1 [.CommandBufferContext]GL ERROR :GL_INVALID_FRAMEBUFFER_OPERATION : glClear: framebuffer incomplete
index.html:1 [.CommandBufferContext]GL ERROR :GL_INVALID_FRAMEBUFFER_OPERATION : glDrawElements: framebuffer incomplete
index.html:1 [.CommandBufferContext]GL ERROR :GL_INVALID_FRAMEBUFFER_OPERATION : glClear: framebuffer incomplete
index.html:1 [.CommandBufferContext]GL ERROR :GL_INVALID_FRAMEBUFFER_OPERATION : glDrawElements: framebuffer incomplete

It's possible this is a bug in Chrome's implementation, but we should check it out either way and file a bug with them if it is.

[pjcozzi]

Works now in Canary 56.

Some Cesium features may not work yet, but we'll handle that as part of #797.