emscripten: don't dispatch user input to hidden windows

[vittorioromeo]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

DISCLAIMER: AI assistance was used to identify and understand the bug. The fix was hand-written.

Summary

When two SDL_Windows share the same DOM canvas on Emscripten (e.g. a visible main window alongside a hidden 1×1 window for a shared GL context -- same setup as #15511), mouse button events end up tagged with the hidden window's ID and are silently dropped by code that filters events on event.button.windowID. Symptom: a single physical click behaves as if held down, because the release event never reaches the application.

Details

Both windows register pointer-event listeners on #canvas; both fire on every event.

Each calls SDL_SendMouseMotion(window_data->window, ...), which calls SDL_UpdateMouseFocus.

The motion coordinates are pre-scaled by window->w / canvas_css_w -- so for the hidden 1×1 window, a real cursor at canvas pixel 800 becomes 0.625, which is inside [0, 1).

SDL_UpdateMouseFocus then sets mouse->focus to the hidden window. The next SDL_PrivateSendMouseButton tags event.button.windowID = mouse->focus->id with the hidden window's ID.

Other SDL backends (Win32, Cocoa, X11, Wayland) get an invariant for free: the OS doesn't deliver input events to non-visible windows. Emscripten has no such invariant -- SDL_WINDOW_HIDDEN is just display: none on a (possibly shared) DOM canvas.

Fix

Two early-returns in src/video/emscripten/SDL_emscriptenevents.c, both gated on the runtime SDL_WINDOW_HIDDEN flag:

• Emscripten_UpdateMouseFromEvent: hidden windows neither update cursor position nor dispatch button events.

• Emscripten_HandleMouseFocus: hidden windows can't claim mouse->focus on pointerenter.

Checking the flag at dispatch time (rather than gating handler registration) means a window that toggles between hidden and shown via SDL_ShowWindow / SDL_HideWindow participates in input correctly while shown, with no re-registration needed.

Testing

Manually verified on Chrome via emrun using the same multi-window setup as #15511.

Notes

This PR is a follow-up to #15511: same VRSFML hidden-window-for-shared-GL-context pattern.

Similarly, this PR is also related to #12512.