Test sampler array using loop index in a separate test.

In WebGL 2, sampler array using loop index is not allowed anyway, and this is a very specific driver bug, so I put a new test under glsl/bugs, with added "break" code to test loop unrolling if it is applied.
Fix a bug
Add a comment and fix a bug.
Merge pull request #480 from zhenyao/loop

Loop
Test struct type matching for uniforms.
Merge pull request #482 from zhenyao/match

Test struct type matching for uniforms.
Strengthen the test case for loop-unrolling.

So the index might not just be a symbol, but could be expression.
Also, see if the index used as function param also get replaced with value while unrolling.

This is added due to bugs in the attempted implementation.
Merge pull request #483 from zhenyao/loop

Strengthen the test case for loop-unrolling.
Updated IDLs with correct 'implements' syntax
Merge pull request #486 from toji/idl_update

Updated IDLs with correct 'implements' syntax