Due to the plugin creating a thread to retrieve and output the
captured image data, care must taken to not modify data the
thread reads from the outside while the thread is running.

In previous revisions some settings accessed by the capture
thread were written to in the update function which could cause
image corruption and in the worst case crashes.

The members of the data struct are now split into two groups,
those that are used by the thread while it is running and must
not be changed from the outside, and those can be changed at any
time.