Cull unnecessary shadow drawLayer calls.

And clearly document what is generated and why these settings are used.

Bug: b/192671494

Test: trace shell start and confirm with perfetto that the set of
      shaders generated has not changed. Also run librenderengine_test.

Change-Id: Ic939d03a26f2c934b3a8fd55ecb9020e9522ecbd
1 file changed