sokol_app.h: support sapp_toggle_fullscreen with HTML5

[edubart]

Currently sapp_toggle_fullscreen is not implemented for HTML5 backend, however emscripten has the functions emscripten_request_fullscreen and emscripten_enter_soft_fullscreen which could be used to implement it.

There is an example if it in https://github.com/emscripten-core/emscripten/blob/main/test/test_html5_fullscreen.c

[floooh]

Yeah I'm aware of those functions (and IIRC actually used them in early sokol_app.h versions), but had a lot of trouble in the past because fullscreen implementation details differ a lot between browsers (and IIRC iOS Safari didn't support fullscreen at all for a long time). One problem I remember was that the fullscreen security popup clashes with mouse-lock security popup which made UX a mess.

There's also left-over comment which hints at some of the problems I ran into:

sokol/sokol_app.h

Lines 5082 to 5101 in d4ac122

	/* The above method might report zero when toggling HTML5 fullscreen,
	in that case use the window's inner width reported by the
	emscripten event. This works ok when toggling *into* fullscreen
	but doesn't properly restore the previous canvas size when switching
	back from fullscreen.
	In general, due to the HTML5's fullscreen API's flaky nature it is
	recommended to use 'soft fullscreen' (stretching the WebGL canvas
	over the browser windows client rect) with a CSS definition like this:
	position: absolute;
	top: 0px;
	left: 0px;
	margin: 0px;
	border: 0;
	width: 100%;
	height: 100%;
	overflow: hidden;
	display: block;
	*/

[edubart]

I see, I managed to workaround by adding some JavaScript code in the web frontend, so this problem can be solved in outside the sokol_app.h. Thus consider this low priority.