This one small change allows the update_glslang_sources.py script to operate correctly with Python 2 and Python 3.

Change the string literal type to "bytes" so that it matches the type returned by the subprocess calls.  Otherwise, under Python 3, the search for "known-good" in the list of remotes always fails.  This is OK for the first execution of update_glsang_sources, since the remote is not there on the first run.  But on subsequent runs, the search still fails to match and the script stops when trying to create a remote that already exists.

- remove unused variable to pass pylint
- Use another approach to detect if known-good remote is already
  present to avoid the need for "ignore following errors" message.

--site gitlab must be added to command for gitlab capability.
Default is github.

Also added known-good mechanism to fetch latest validated spirv-tools.
Also added -Od and -Os to disable optimizer and optimize for size.

Fetching spirv-tools is optional for both glsl and hlsl. Legalization
of hlsl is done by default if spirv-opt is present at cmake time.
Optimization for glsl is currently done through the option -Os.

Legalization testing is currently only done on four existing shaders.
A separate baseLegalResults directory holds those results. All previous
testing is done with the optimizer disabled.