If glTexBuffer is used, the width should be calculated based on the
buffer size.  Since ES doesn't have immutable buffers,
EXT_texture_buffer allows mutable buffers to be attached to the texture.
This means that updates to the buffer must update the image description.
This is done by having the texture observe the attached buffer.

Bug: angleproject:3573
Bug: angleproject:5334
Test: dEQP-GLES31.functional.state_query.texture_level.texture_buffer.width_*
Test: TextureBufferTestES31.QueryWidthAfterBufferResize
Change-Id: I7d9e5344a3d4ea70da7ba525f13b5876481a70a3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2533495
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>