A DispatchHelper class is created as the equivalent of FramebufferHelper
as a command graph resource.  There's currently a single dispatcher and
all dispatch calls are recorded on that.  Context dirty bits are set up
in such a way that graphics and compute workloads are independently
handled, so that issuing a dispatch call wouldn't cause a framebuffer's
render pass to rebind resources.

Bug: angleproject:3562
Change-Id: Ib96db48297074d99b04324e44b067cfbfd43e333
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1688504
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>