MAC - Full screen, correct sizing

**Is your feature request related to a problem? Please describe.**
i'm always frustrated when i'm trying to enter in fullscreen mode and when i'm trying to resize the window of the scrcpy size, it quits the mode. currently it's like this [current state](https://ibb.co/WBxXkrq). The main issues are the black areas.

**Describe the solution you'd like**
Ideally i would like to be like that : [ideal state](https://ibb.co/NN2vYCg) but in fullscreen mode.

**Describe alternatives you've considered**
Being not in fullscreen mode but it's not effective because i always want to have `scrcpy --always-on-top` and i dont want to resize all my apps for that
**Additional context**
Not relevant

Start by reproducing the issue on macOS with fullscreen resizing and the --always-on-top option, then trace the existing fullscreen and window-sizing behavior. Done means fullscreen resizing no longer exits the mode and the window displays with correct sizing without black areas.

Written by the indexing model from the issue text.