Allow setting optix maxTraceDepth value through API function

[ingowald]

Currently OWL is hardcoded to a trace depth of 2, which is
a) limiting (for users that want to use more); and
b) wasteful (since most of our apps and samples only use 1).

Suggest

• set default trace depth to 1
• add api function to set it to something larger, similar to how we set other contet/pipeline constants before pipe creation.