Canvas size fixes in Emscripten mode

[Grey41]

Description

In the old code, we tried to detect whether the canvas size was controlled with css by setting the canvas size to 1, then using emscripten_set_element_css_size to see if anything changed.

I suggest we leave the canvas size alone, except when the SDL_WINDOW_FILL_DOCUMENT flag is set. This means the canvas doesn't unexpectedly mutate (or its css). Also, on destroy, it's best just to clean up and not to mutate the canvas.

Finally, when SDL_WINDOW_HIGH_PIXEL_DENSITY exists, work backwards from the canvas size. For example:

window.width = canvas.width / pixelRatio
window.height = canvas.height / pixelRatio

This is intuitive because the canvas is setting the precedent for the window size, instead of SDL trying to guess by peering into the css.

[slouken]

Conceptually this seems like a good idea. @icculus?

[icculus]

Not against it, but I'm real nervous to apply this right before 3.4.0 ships.

[slouken]

I'll bump this to the 3.4.2 milestone so it can get some more testing.