If a texture level is being redefined, there are two scenarios to
consider:

1. The level is outside the base/max level, for which the image was
  allocated.
2. The level is within the base/max level, but it's being redefined to a
  different size or format.

In the former case, we simply don't need to release the image.

The latter case itself has two possibilities:

2.1. There is only one level in the image.
2.2. There are multiple levels in the image.

In case 2.1, the whole image is being redefined (as it has only a single
level), so the image can (and should) be released.  Prior to this
change, this behavior was adopted for all cases.  This change retains
this behavior for this case only.

In case 2.2, the texture is becoming incomplete.  However, the image
shouldn't yet be released because another one of its mips may be bound
to a framebuffer.  In such cases as glCopyTexImage2D(), that framebuffer
may in fact be the source of the copy operation (which would be
destroyed if the image is released).  If the base/max level of the
texture doesn't change, redefining the level and making the texture
incomplete doesn't make the framebuffer incomplete; this is achieved at
the same time by not releasing the image.

This change ensures that updates to the redefined level are staged in
cases 1 and 2.2.

Bug: angleproject:4274
Change-Id: I3fac3203c2fbbc16e8e4a35b1334b767120b2dcf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2230853
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>