Update SDL_PROP_WINDOW_EMSCRIPTEN_FILL_DOCUMENT_BOOLEAN when SDL_HINT_EMSCRIPTEN_FILL_DOCUMENT changes

[slouken]

[icculus]

Ohhh, this actually brings up a thing I've been thinking about: should we notify objects when their properties change?

If we do that, the hint goes back to being used for window creation, but apps can toggle it on specific windows by changing the property, which will fire an optional callback.

[icculus]

(This isn't crucial for this specific thing: there's only one Emscripten SDL_Window at a time at the moment. But the idea still stands.)

[slouken]

I've thought about this a lot, and I don't think that we should introduce the concept of object property notifications or callbacks when properties are changed. That's a slippery slope that we can slide down later if we need to.

However, it does seem reasonable to add a window function for this and a window flag and event when this changes. I feel like this is a core enough feature of Emscripten that it's worth calling out as a window mode.

[icculus]

I've thought about this a lot, and I don't think that we should introduce the concept of object property notifications or callbacks when properties are changed. That's a slippery slope that we can slide down later if we need to.

Yeah, it'd be a serious rabbit hole to fall down...let's leave it alone for now (or forever maybe).

However, it does seem reasonable to add a window function for this and a window flag and event when this changes. I feel like this is a core enough feature of Emscripten that it's worth calling out as a window mode.

Your wish is my command: #14619.