linux-v4l2: Fix select in capture thread.

This moves the enabling/resetting of the file descriptors inside the
capture loop so it is done before each select call. Why this even worked
before is unclear, but doing it the *right* way seems to reduce latency.
linux-v4l2: Make timestamp offset local.

This replaces the var in the source struct that are handling the
timestamp offset with a local one in the capture thread.
This change is mostly to make the code more readable.
linux-v4l2: Make frame counter local.

This replaces the var in the source struct that was used to print out
the number of captured frames with a local one.