vulkan: Don't merge too many drawing commands

vulkan: Don't merge too many drawing commands

If a command takes too long to execute, Vulkan drivers will think they
are inflooping and abort what they were doing.

For the simple color shader with smallish nodes, this happens around
10M instances, as tested with the output of
  ./tests/rendernode-create-tests 10000000 colors.node

So just limit it to way lower, so that we barely never hit it, ut still
pick a big number so this optimization stays noticable.