Fix physical vs logical size confusion

On a computer with a scale factor different from 1, physical and logical
sizes differ. For example, with a scale factor of 2, if the logical
(window) size is 800x600, the physical (renderer) size is 1600x1200.

They were not interpreted consistently in scrcpy.

To fix the confusion:
 - resize the flex display according to the logical size
   (SDL_EVENT_WINDOW_RESIZED);
 - scale rendering to match the physical area defined by the logical
   size;
 - no longer convert input event coordinates.

This was not an issue before flex displays because rendering scaled the
content to fit the window (`--render-fit=letterbox`), so the difference
in physical size had no effect.

Refs #6772 comment <https://github.com/Genymobile/scrcpy/pull/6772#issuecomment-4360006349>
PR #6772 <https://github.com/Genymobile/scrcpy/pull/6772>