Instead of using a temporary pointer-to-pointer, try casting the
pointer-to-pointer type directly.

This will facilitate Vulkan's use of RenderTargets.

BUG=angleproject:1319

Change-Id: I0a14762e8e442b9eaa6eba3fdf653aef3c362533
Reviewed-on: https://chromium-review.googlesource.com/424148Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>