Prevent closure compiler from mangling WebGL 2 API calls

[mosra]

Fix for #3473 . I just went through all GLctx.something(...) calls in the WebGL 2 section, replaced them with GLctx['something'](...), tested some of these with --closure 1 and they now work. The only places where I left the original code are in makeSetValue() calls like this one:

{{{ makeSetValue('params', '0', 'GLctx.getSamplerParameter(sampler, pname)', 'float') }}}

I don't know how the closure compiler handles these (if at all) so they are left untouched. Should I change them too?

[juj]

Closure compiler will not see the makeSetValue commands, themselves but those are expanded to the expression in the third parameter before the output is passed to closure. Therefore it is possible that the GLctx.getSamplerParameter() call from inside that expression would get closure transformed to something like `GLctx.eA'. Replacing those as well makes sense.

[mosra]

Done.

[juj]

Thanks!