RenderTarget11 knows how to render to a specific mip of a texture but if
TextureStorage11 attempts to read or write to the backing resource directly
it does not select the correct subresource index.  This manifested in calls
to glTexSubImage going to the wrong mip of the resource.

Fix TextureStorage11::getSubresourceIndex to look up the correct subresource
in TextureStorage11_EGLImage.

Add a test to cover this case in ImageTest.MipLevels.

BUG=angleproject:2668

Change-Id: Id9270ab3bf9f37c7c671639bb8b7ce9578daaed6
Reviewed-on: https://chromium-review.googlesource.com/c/1422058Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>