Minimum changes to glsl-sandbox to get WebGL 2.0 working

[Spaxe]

Hello,

This is a set of changes to get WebGL 2.0 going in glsl-sandbox. You can see the result here: https://spaxe.github.io/glsl-sandbox/

Caveats:

• Fragment Shader must be written with #version 300 es. This can be detected and change the vertex shader to fit. The program now detects WebGL version in browser. Could be smarter about shaders, though.
• I haven't tested it with the Ruby setup.
• Haven't looked at core/extensions.

Changes:

• Backwards-compatible WebGL context detection.

gl  = canvas.getContext( 'webgl2', contextAttributes )
   || canvas.getContext( 'webgl', contextAttributes )
   || canvas.getContext( 'experimental-webgl', contextAttributes );

• Updated example shaders in static index.html page to GLSL ES 300.
• No change to server folder.
• Updated default shader to GLSL ES 300.

[TheLucifurry]

That PR is still opened)