RemoteDesktop: Fix relativity of absolute pointer motion

According to the interface, the given pointer coordinates are in logical
coordinates relative to the specified stream. We were completely
ignoring that, instead having a global "streamed screen position", which
was set to whatever the last stream was that was started. This breaks
when using multiple streams, causing all input to be relative to the
last stream.

To fix this, add a "geometry" getter to ScreencastingStream that gets
set to the stream's geometry when created. Then, in
requestPointerMotionAbsolute, we lookup the requested stream and use
it's topLeft as offset to the provided mouse position.