Fixed GLES3.0 attribute locations

Previously, it was allowed for multiple attributes to share a
location or overlap, but in ES3.0, it should cause a linking error.

Passes WebGL conformance tests.

Change-Id: I1c65b0cb31a04680a25ac54d20f6ee0d869251b9
Reviewed-on: https://swiftshader-review.googlesource.com/4877
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>