This implements the following suggestions from Adam Sawicki:
(1) Update VkMemAlloc to dev branch.
(2) Use VMA_MEMORY_USAGE_CPU_ONLY in the stage pool.
(2) Add vmaFlush after every vmaUnmap.
We also considered removing Map / Unmap in favor of persistent mapping
via VMA_ALLOCATION_CREATE_MAPPED_BIT, but decided to be conservative
until we have a chance to rethink our UBO update strategy.
Closes#19
7fff7d715f