add trackpad pinch

- [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.**
I'm always frustrated when I try to "pinch" on my trackpad, and it does nothing on the android screen.

**Describe the solution you'd like**
I want to be able to pinch on the macbook trackpad, and it respect that on the device.

**Describe alternatives you've considered**

ctrl+mouse works, but requires using the keyboard...

Start by finding the existing ctrl+mouse pinch handling and the trackpad input path. Define how the MacBook trackpad gesture should translate to Android, then verify the behavior on a trackpad; the issue names no files or tests, so the implementation and validation points must be established first.

Written by the indexing model from the issue text.