This makes the current clear color a bit more accessible than using
the current GL state clear color directly. It is stored in the Context
using the Vulkan clear color struct.

This also cleans up the attachment clear values in FramebufferVk and
paves the way for depth/stencil clear when support for depth/stencil
exists.

Bug: angleproject:2357
Change-Id: I0553e4447249325115625a44e9f4b1623c7cb0bc
Reviewed-on: https://chromium-review.googlesource.com/919550
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>