* Export desktop recordings to video
Add a new tsh command that will write Windows desktop recordings
to an AVI file for offline playback. Encoding is done client side
to avoid consuming server resources.
This uses the Motion JPEG codec (https://en.wikipedia.org/wiki/Motion_JPEG)
for its simplicity and ease of use. Something like ffmpeg would perform
better in nearly every aspect (run time, compression / file size, video
quality, etc), but that would complicate our build process and add extra
native dependencies. This implementation uses pure Go and works on any
platform where tsh runs today.
Also make sure `tsh recordings ls` shows Windows and SSH recordings.
* Untangle test imports
lib/events/eventstest is allowed to import lib/events
(it needs to in order to implement interfaces and use types)
This means lib/events can not import lib/events/eventstest,
which requires that we move some tests from package events
to package events_test
* tdp: break dependency on lib/srv
The lib/srv package is large and contains Unix-specific code.
Now that tsh needs to understand the TDP protocol, we need to
avoid importing lib/srv so that tsh can still build on Windows.
