magic trackpad two finger swipe to simulate back

- [x] I have checked that a similar [feature request](https://github.com/Genymobile/scrcpy/issues?q=is%3Aopen+is%3Aissue+label%3A%22feature+request%22) does not already exist.

**Is your feature request related to a problem? Please describe.**
In google chrome , two finger swipe left / right using magictrackpad can lead to a back/forward behaviour, its convenient ,but scrcpy did not support this feature

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
two finger swipe left to back

Start by locating scrcpy's host input-handling entry points and its SDL2 event processing, then check how trackpad gestures are currently represented. Determine the supported back-navigation behavior and identify the tests or manual checks needed to confirm that a two-finger swipe is handled consistently.

Written by the indexing model from the issue text.