[web] Fix pointer events for Wacom pen

[mdebbar]

When a mouse is being used, we always seem to be sending -1 as the pointer ID to Flutter. But the logic that tracks the up/down states is using the real pointer ID (the one that the browser gives to us, which is usually 1, but sometimes could be random as described below).

In some devices, like Wacom, the pen events are handled by the browser as if they were mouse events. But the pointer ID seems to be randomly generated (i.e. pointerdown is received with pointerId=21, and pointerup is received with pointerId=45 even though it's the same pen, see example).

By making sure to always use -1 for mouse, we will fix both issues at once.

Initially, I thought this could cause issues when there is more than one mouse attached to the device. But it turns out that the browser assigns the same pointer ID to all the attached mice. So we can also do the same.

Fixes flutter/flutter#75559

[dkwingsmt]

LGTM

[dkwingsmt]

I don't think any major operation system is supporting multiple mouse cursors (i.e. all of them combines the movement of multiple mice into one cursor). Until we meet real use cases, we should be safe to assume we're only supporting one mouse cursor.