Add --render-fit feature

Add an option to configure how the rendering fits the window.

The default, `--render-fit=letterbox`, preserves the aspect ratio
and fits the window as best as possible, adding black bars at
the top/bottom or left/right if needed. This has been the only behavior
scrcpy supported so far.

Another mode, `--render-fit=unscaled`, renders the display without
scaling. This mode will be useful for virtual display resizing.

Refs #6772 comment <https://github.com/Genymobile/scrcpy/pull/6772#pullrequestreview-4250571943>
PR #6772 <https://github.com/Genymobile/scrcpy/pull/6772>