Show translated shader source using WEBGL_debug_shaders in glsl tests

The translated shader source can be useful when debugging shader
translator or GPU driver issues, so display it alongside the original
source if WEBGL_debug_shaders is available.

Also includes some cleanup of related code.
Refactor shader logging to reduce code duplication

Some tests that did not include shader logging before also get shader
logging.
Merge pull request #514 from NVIDIA/use-debug-shaders

Show translated shader source using WEBGL_debug_shaders in glsl tests