Downsize on error only before the first frame

The purpose of automatic downscaling on error is to make mirroring work
by just starting scrcpy without an explicit -m value, even if the
encoder could not encode at the screen definition.

It is only useful when we detect an encoding failure before the first
frame. Downsizing later could be surprising, so disable it.

PR #2947 <https://github.com/Genymobile/scrcpy/pull/2947>