The patch enables viewport selection for multiview rendering in the
GLSL/ESSL vertex shader through the use of the GL_NV_viewport_array2
extension. The AST is modified only for GLSL and ESSL to include the
viewport selection expression after ViewID_OVR's initialization.

BUG=angleproject:2062
TEST=angle_unittests

Change-Id: Iee05bb5a4b687ed53ddbdd466f1572227b1f0cde