Proposal: `active_pointer` flag
## The Problems

I feel current mouse and touch input leaves things to be desired. These are the two most **critical problems** in my mind:

1. The mouse position remains at the last touch position. This instantly gives off the feeling of a poorly done port to a touch device and the cart itself cannot do anything about it.

2. The mouse position updates when the cursor is somewhere else on the web viewport but remains at the last on-viewport position when leaving the viewport. A cart could avoid off-canvas positions and unify the experience of leaving the position at the last on-canvas one. But again I don't think leaving things were they last were is a good solution.

## Proposed Solution
The introduction of a `active_pointer` flag would solve both those problems. With a mouse it would only be `true`, when the mouse is withing the 160x160 pixels, otherwise `mouse_x` and `mouse_y` are meaningless. On a touch device `active_pointer` would be `true`, when the user is actively touching the screen. See [MDN's definition of *active pointer*](https://developer.mozilla.org/en-US/docs/Web/API/Pointer_events#active_pointer).

A bit on the [`MOUSE_BUTTONS`](https://wasm4.org/docs/reference/memory#mouse_buttons) byte could hold this flag. Because it then wouldn't contain only button info anymore, it could be renamed to something like *mouse flags* or *mouse info*.