Respect encoder minimum size constraint

Encoders require a minimum video size.

Use video capabilities to constrain a given size by the declared minimum
size of the selected encoder.

Contrary to the maximum size constraints (but like the alignment
constraint), use the same value in both directions for simplicity and so
that rotating the device does not change the shape of the video.

PR #6766 <https://github.com/Genymobile/scrcpy/pull/6766>