Fix wasm32 deployments not displaying anything

[derezzedex]

This fixes a bug where web deployments of an iced application would silently fail, not displaying anything.

The problem can be tested by using trunk serve in any of the examples that support it.

It seems to have started after #2469, where changes were introduced to allow an application to be started without an window. Although an window is not necessarily shown at the start, it's still created silently, and this is necessary for the acquisition of the graphics device and it's initialization, after that the window is discarded and only created again when requested by the runtime (or quickly after that on "non-deamon" applications).

On the wasm32 path of the code, the window contains the HTMLCanvasElement, which in turn, has the getContext method and contains the WebGLRenderingContext which is necessary for properly displaying graphics on the canvas.
By discarding the window, we end up discarding the HTMLCanvasElement which is associated with the context and other graphics dependencies for proper rendering; creating a new window without using the correct associated canvas element which was used to initialize the context, then makes the canvas graphically display nothing.

There could multiple solutions to this, but the simplest seems to be to simply retain the canvas element, and re-use it to create the window through the with_canvas window attribute.

This PR changes the wasm32 path to store the web_sys::HTMLCanvasElement and use it on the with_canvas method when creating an window, I've also added logging for the window attributes to the relevant code, which could be helpful in future debugging of similar problems.

As for possible problems with this, the code doesn't make any guarantees for other windows, it'll certainly silently fail when asked to create new windows, since those are basically canvas elements on wasm32 that are not properly created or maintained. Although I would add that this was already the case before the PR.

Closes #2480.

[hecrj]

Sweet!

Made some changes to discard the empty polling events during boot.

[derezzedex]

Sweet!

Made some changes to discard the empty polling events during boot.

Nice catch!