Serializes textures' states stored on CPU.
Gets textures' contents at every mipmap level from GPU then
serialize them.

Adds texture serialization to serializeContext method so that capture
replay regresssion testing now compares the states of texture too.

Bug: angleproject:4817
Change-Id: I87670fdc744f96d4ff244462dee1a065de44f9b7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2325029
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>