This enables ReadPixels from non-0 FBO attachments.

Tested with:
dEQP-GLES3.functional.negative_api.buffer.read_buffer
 (fails due to a possible error in dEQP)
dEQP-GLES3.functional.shaders.fragdata.draw_buffers
 (passes)

BUG=angle:903

Change-Id: Ib095e1d50b8fbbc6f197de25f19ec0993309145a
Reviewed-on: https://chromium-review.googlesource.com/244841Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>