[HTML5] Fix input not working when buffered.

[Faless]

After input buffering was reworked, input accumulation is now handled outside of OS, and the JavaScript plaform never implemented that.
Additionally, the JavaScript platform is quite obnoxious about calling specific APIs outside specific user triggered events.

This commit adds event flushing during the main iteration, and forces it during keydown/keyup/mousedown/mouseup/touchstart/touchend/touchcanel events (effectively only accumulating only "move" events).

Fixes #52468.
3.x version: #52603

[mhilbrunner]

Would it make sense to do this only in process_events() and additionally add a conditional that does this inside input->parse_input_event()? Or add a setting to input like "flush after every event" that is enabled for JS, and then input->parse_input_event() flushes on each call?

Reason being
1.) this seems messy to have in 3 places that generate input events,
2.) this seeming error prone if we add more places that generate input events, because the implementer always needs to remember to also add the flush.

Then again, that might be overkill. Besides this, this looks good and to be the correct fix.

[Faless]

Or add a setting to input like "flush after every event" that is enabled for JS, and then input->parse_input_event() flushes on each call?

The main benefit from accumulation is in the handling of move and drag events. Which can be fired very rapidly.

I understand having the flush in the callbacks is a bit messy, but it's unlikely we'd add more events of that kind, because those really are the only relevant actions. I.e. we don't want to flush on every event, we only want to flush on clicks/press so we can interact with the APIs that requires direct user input.

EDIT: I also have a PR that unifies the key callbacks, which will save some more duplication.

[mhilbrunner]

LGTM