OpenGL ES environment texture partial implementation

Change-Id: I0dcce9f2659cfdde8cc4d5a0a541bea7728b3f97
Reviewed-on: https://swiftshader-review.googlesource.com/1850
Tested-by: Maxime Grégoire <mgregoire@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>