Vulkan: Clean up FramebufferVk blit methods. Renames mReadPixelBuffer to make it consistent with the blit buffer. Also moves the rectangle clipping out of the blit implementation methods. This shares a bit of code. Also renames some blit implementation methods for consistency. Bug: angleproject:2729 Change-Id: Ida81e85af4751cf8cb4b3029ed4e4b53bfa7b03d Reviewed-on: https://chromium-review.googlesource.com/1142298 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>