Updated get.webgl.org to recognize the current state of IE and Opera

The current get.webgl.org doesn't list IE or Opera as having WebGL implementations. Any Opera links that were present pointed to Opera Next, which is no longer necessary for WebGL. Updated the browser list to account for the current state of affairs.

Also added some logic (borrowed from the "context/methods" conformance test) to see if the WebGL implementation advertises support for all required functions. If not, shows a warning that the WebGL implementation is incomplete.