cannot paste into text fields from mobile safari

[psifertex]

Using the demo on an iphone you cannot paste into text fields.

First observed in tpscube but appears to be an egui issue since it reproduces in the demo.

I realize this is potentially hard to work-around given the canvas backing, but one potential work-around is to optionally render a browser native text-field (and getting the exact style correct 😬)

[parasyte]

Mobile IME was added in #253 - it manipulates the visibility and position of an HTML input element to make the IME keyboard show up on mobile devices. I'm not sure how feasible it is to get the hidden input element to notice long-presses for copy-paste, but it's a starting point.