Ozone only supports external target for images created with
EGL_EXT_image_dma_buf_import.

BUG=angleproject:4443
BUG=chromium:1063462

Change-Id: Ia62fdd92198bdcc2d9840765305b4cc613fd3f7c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2113450Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>