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 sampler type.

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

Signed-off-by: Yevhenii Kolesnikov

Edited by Yevhenii Kolesnikov

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