add trackpad pinch

[rdp]

• [x ] I have checked that a similar feature request 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...

[rom1v]

Probably requires to use SDL_MultiGestureEvent.

I never used pinch-to-zoom on my touchpad (in fact I never use a touchpad), and apparently it does nothing special on my laptop, so I could not test.

[ballo]

https://github.com/libsdl-org/SDL/pull/9445#issuecomment-3395389283