Fixed a WebGL conformance test

It is illegal to use both glFragData and glFragColor in the same
shader according to the OpenGL spec, so a check was added to
verify that this doesn't happen, and returns an error if it does.

BUG=18029434

Change-Id: Ifee14dd539cacc7cd16a2c2634d3e021dbf7ff8e
Reviewed-on: https://swiftshader-review.googlesource.com/1430
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>