sokol_app.h emsc: more control over input event consumption

[floooh]

Only on the Emscripten platform:

• mouse and touch events are now by default passed up to the web page and can be consumed by calling sapp_consume_event() from within the event handler
• wheel events are still always consumed (passing them up tries to scroll the webpage which looks bad even with a 100% canvas)
• no change (yet) for keyboard event behaviour, but probably a good opportunity to tackle https://github.com/floooh/sokol/issues/824)