Erroneously reports some browsers as supporting WebGL2

Some browsers (Safari) you can enable WebGL2 but in reality they haven't actually implemented WebGL2. AFAICT all Safari has done is (a) respond to `getContext('webgl2')` and (b) allow GLSL ES 3.0 shaders. All other functionality over WebGL1 does not work. Running the webgl2 conformance tests around 80% of them fail.

What do you think about trying to detect that situation? At the moment a simple test is to test if some of the parameters returned are at least the required minimum values. Safari for example returns both less than minimums and various INVALID_ENUMs for things they haven't yet supported.