This is a first step in handling interface blocks
properly by allowing indexing of blocks to be handled
properly. This does not include any interaction with
the Program, which will come in a following step.

No WebGL tests were injured in the making of this cl.

Change-Id: I575b08502f7f4dd63ca7b57f2ca0630367adf0de
Reviewed-on: https://swiftshader-review.googlesource.com/3589Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>