GL support detection broken

[edenh]

Ref #664

After disabling WebGL disabled in Safari 8, the detection script is returning true.

[ansis]

window.WebGLRenderingContext exists even if webgl is disabled. It looks like the only reliable way to know for sure is to canvas.getContext and check if it succeeded. Our options:

• create and discard a context
• create a context and keep it around for the map to use
• remove supported and have map creation fail if webgl is not supported

[jfirebaugh]

create and discard a context

This.