The stack is arbitrary strings that the validation layer debug callback
prints in reverse order after each message. This makes it easy to work
out what nodes in a pipeline/render pass plist are causing validation
errors. Still have to narrow down the actual line, but the messages seem
to help with that.
Putting qfvPushDebug/qfvPopDebug around other calls to vulkan should
help out a lot, tool.
As a bonus, the stack is printed before debug_breakpoint is called, so
it's immediately visible in gdb.
