Restrict GLSL globals to be initialized with constant expressions.

Restrict GLSL globals to be initialized with constant expressions.

This follows the GLSL ES 1.0 and 3.0 specs more strictly.

Change-Id: I323e90ef0a1588109e2cb7988136a9520e501a6d
Reviewed-on: https://swiftshader-review.googlesource.com/5065
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>