added additional check to see if WebGL is supported

I ran into this issue on Chrome for Linux -- if WebGL is supported but not *fully* supported with OpenGL, it would return "null" on `canvas.getContext("webgl")` instead of "undefined". The original check in the Babylon.js code would pass and Chrome would throw an error about `MAX_TEXTURE_IMAGE_UNITS` not being defined and wouldn't render anything. This simple additional if clause fixes that issue.