glsl: Enable textureSize for samplerExternalOES

From OES_EGL_image_external_essl3

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1901



Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Tapani Pälli <tapani.palli@intel.com>