Use is_icon flag to compute content rectangle

The content rectangle was computed differently depending on whether
`can_upscale` was set. In practice, this flag was set when the content
was a frame (not an icon).

Instead, use an explicit `is_icon` flag, because the behavioral
differences will go beyond whether the content can be upscaled.

PR #6772 <https://github.com/Genymobile/scrcpy/pull/6772>