try and create webgl2 context with webgl1 as fallback

[heilmoli]

• I propose to take advantage of webgl2 where available (shadertoy does this too). I think this makes sense as there are feature differences between platforms anyway.
• Enabled OES_standard_derivatives by default (still needs to be enabled in the shader itself)
• Fixed a bunch of unexpected type coercion warnings in gl.js.

[not-fl3]

Hi, thanks for PR, great work!

Just opened an issue to discuss how we can make it possible: https://github.com/not-fl3/miniquad/issues/176