If a texture is not yet initialized then it may contain random
pixels, and since it doesn't make sense to capture this random
data, skip its serialization and only mark the data as not
initialized in the capture.

This also avoid the texture syncronization that would lead to a crash
for incomplete cube maps.

Bug: angleproject:6034
Change-Id: I0cb30a03a29aecd605f3c61b6df4cd897e1f9093
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2944954
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>