Line loops aren't supported in Vulkan directly, so we use line strips with
an indexed buffer to emulate them. To hide the complexity of that, I've created
the LineLoopHandler class in vk_utils.

Bug: angleproject:2335

Change-Id: Id3e020d27e5265565e61e96d3fd0187c4fe2b152
Reviewed-on: https://chromium-review.googlesource.com/931421
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>