glBindImageTexture specifies the format which should be used to
interpret the texture data and this format is independent from the
texture's own internal format. This change allows the VkImage's format
to be mutable to handle glBindImageTexture calls with different
formats.

Bug: angleproject:3885
Test: dEQP-GLES31.functional.image_load_store.*.format_reinterpret.*
Change-Id: Ia1ad762b4ccae0f510c8b4918781234fcf51c5f1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2222610Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>