Draft: tracer: add stateless tracer and retracer utilities

The tracer and retracer utilities help to test v4l2 stateless decoder drivers by tracing, recording and replaying (i.e. "retracing") userspace's interaction with a stateless decoder driver. The tracer follows a userspace application and records v4l2 system calls in a json file. The retracer reads and replays the json text file. The retracer can be run in a different environment than the tracer. The json text file can be edited (e.g. to inject errors) before it is run by the retracer.

Signed-off-by: Deborah Brouwer deborah.brouwer@collabora.com