This CL adds support for dumping the VMA stats string, which can be
given to VmaDumpVis.py to visualize the allocations that the VMA has
performed.

To enable dumping the string, set:
RendererVk.cpp
  rx::kOutputVmaStatsString = true

Copy the desired JSON output into a text file, and pass that to
VmaDumpVis.py:
python3 \
  third_party/vulkan_memory_allocator/tools/VmaDumpVis/VmaDumpVis.py \
  -o stats.png stats.txt

The legend for the visualization is available at:
https://github.com/GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator/tree/master/tools/VmaDumpVis

Bug: angleproject:2162
Test: Manual verification
Change-Id: Ic8c1002805dd57e594df724bcf1cdbc1d1599a3e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2472525Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>