main: allow external textures for BindImageTexture

From issue 10 of the OES_EGL_image_external_essl3:

  A limited set of use-cases is enabled by making glBindImageTexture
  accept external textures. Shaders can access such external textures
  using the existing <image2D> sampler type.

Fixes: 02a6d901 ("mesa: add OES_EGL_image_external_essl3 support")

Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit ed43dd62)