Emscripten: Allow for Multiple Windows

[Temdog007]

• Updated the software renderer to store each canvas and context in a separate object
• Updated drag and pointer event handlers to store each event listener function in a separate object
• Added hash table to store which canvas ID has been mapped to an SDL_Window
• Updated Emscripten_GLES_MakeCurrent to point Module['canvas'] to the canvas of the renderer's window that is currently being drawn to
• Allow SDL tests applications to show multiple windows on Emscripten builds
• Detect if the canvas doesn't already exist in the DOM and create and manage the canvas if it doesn't
• Add support for SDL_ShowWindow, SDL_HideWindow, and SDL_SetWindowPosition for Emscripten windows

Closes #12512

[sezero]

Also, can you please do:

git pull --rebase https://github.com/libsdl-org/SDL.git main

.. and then do a git push --force

Because those merge commits are really annoying.

[vittorioromeo]

Possibly related to #12678 and #12667 (due to SDL_GetWindowPosition)

[madebr]

I think it would be useful to add multi emscripten window support to at least one test. (Or create a new test?)

[madebr]

I think this test needs a custom HTML shell file: https://emscripten.org/docs/compiling/Deploying-Pages.html#build-files-and-custom-shell

[madebr]

emscripten support files can be stored in test/emscripten.

[Temdog007]

This test is not specific to Emscripten. It works on desktops and also tests multiple things:

• Creating multiple windows
• Unique rendering to each window
• Getting the global mouse position and spawning a window at that position
• Destroying windows

[madebr]

That's good, but without multiple HTML items to show windows, the test won't work on emscripten.

[Temdog007]

I've verified this PR with this test on Emscripten. So, it does work.

In this PR, a new canvas element is created in SDL_CreateWindow if the canvas ID specified wasn't already present in the DOM.

[slouken]

Does creating a hidden window with OpenGL context work the same way on other platforms? e.g. would this PR fix #12678?

[Temdog007]

Does creating a hidden window with OpenGL context work the same way on other platforms? e.g. would this PR fix #12678?

I left a comment for the author of the bug to verify.

[vittorioromeo]

I've done some quick testing but this still seems broken. Mouse position is still offset when using

Vector2i getPosition(const WindowBase& relativeTo)
{
    return getPosition() - relativeTo.getPosition();
}

and now my game also seems to crash when recreating the HTML canvas via a window resize.

[vittorioromeo]

OK, I've done some further digging and the crash seems to happen because SDL_EVENT_WINDOW_RESIZED is unexpectedly 0x0 (data1 is 0, data2 is 0)

[Temdog007]

OK, I've done some further digging and the crash seems to happen because SDL_EVENT_WINDOW_RESIZED is unexpectedly 0x0 (data1 is 0, data2 is 0)

The window size wasn't being updated in SDL_CreateWindow or SDL_SyncWindow. So, the width and height was left at 0. This has been fixed.