Incorrect mouse button events are sent for "unsupported" mouse buttons

[CendioHalim]

Describe the bug
Using an unsupported mouse button, such as forward/back, can sometimes send an incorrect mouse event to the server. This only happens on chromium and safari.

On chromium, back will trigger button number 3 (scroll up), and forward will trigger button number 4 (scroll down).

On safari, it seems like any "extra" button will return button number 2, which is the same as a middle mouse click.

To reproduce
With chromium:

1. Connect to a server & open an app that listens for scroll events
2. Press back/forward on your mouse
3. Button press will trigger a scroll up/down.

With safari:

1. Connect to a server & open an app that listens for middle mouse buttons.
2. Press back/forward on your mouse.
3. Button press will trigger a middle mouse button click.

Expected behavior
Pressing forward/back should not trigger incorrect mouse button events.

[samhed]

This was fixed as part of merging #1919.