Convert screen encoder to async processor

Contrary to the other tasks (controller and audio capture/encoding), the
screen encoder was executed synchronously. As a consequence,
scrcpy-server could not terminate until the screen encoder returned.

Convert it to an async processor. This allows to terminate on controller
error, and this paves the way to disable video mirroring.

PR #3978 <https://github.com/Genymobile/scrcpy/pull/3978>