Make rendering backend configurable. #9122

Job	Run time
examples-rl (e10_drive)	8m 53s
examples-rl (e11_platoon)	4m 57s
benchmark (e10_drive)	5m 22s
benchmark (e11_platoon)	2m 57s
base-tests (./cli)	3m 4s
base-tests (./envision)	5m 17s
base-tests (./smarts/core --nb-exec-timeout 65536 --ignore=./smarts/core/tests/test_notebook.py)	5m 49s
base-tests (./smarts/env --ignore=./smarts/env/tests/test_rllib_hiway_env.py)	5m 49s
base-tests (./smarts/env/tests/test_rllib_hiway_env.py)	4m 3s
base-tests (./smarts/sstudio)	3m 0s
base-tests (./smarts/ray)	3m 35s
base-tests (./examples/tests/test_examples.py)	7m 41s
	1h 0m 27s

Provide feedback