Vulkan: Quick rename of member variable in DynamicBuffer The new name mNxtAllocationOffset instead of mNextWriteOffset makes more sense. Also merged the 2 offset tracking vars since the user of DynamicBuffer should know to flush or invalidate correctly. Bug: angleproject:2480 Change-Id: If7a04b633794484ad88e73fd6f802fcfe0e3705a Reviewed-on: https://chromium-review.googlesource.com/1069107 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Luc Ferron <lucferron@chromium.org>