Device::UpdateWindow contract is backend-dependent when called while rendering is enabled

[bghgary]

Device::UpdateWindow silently no-ops on some backends and re-binds on others when called while Bgfx.Initialized == true. No error, no log. Asymmetric with #1689 (which made UpdateDevice throw — clean because UpdateDevice is uniformly init-only on every backend).

Mechanics

Update* methods set Dirty on Bgfx.InitState. Each frame, DeviceImpl::Frame() → UpdateBgfxState() runs bgfx::setPlatformData + bgfx::frame(BGFX_FRAME_DISCARD) + bgfx::reset when dirty. The new nwh reaches g_platformData; whether bgfx re-binds the swap chain / surface is backend-specific:

Backend	Re-binds to new nwh?
OpenGL / Android	Yes (GlContext::resize recreates EGL surface from g_platformData.nwh)
OpenGL / Emscripten	Yes (canvas resize)
OpenGL / Linux Wayland	Resizes existing window only
OpenGL / Linux X11 (GLX)	No
D3D11 / Win32	No (swap chain bound to original HWND at init)
D3D11 / UWP SwapChainPanel	Yes, when m_scd.ndt == SwapChainPanel marker
D3D12	No
Metal	Only when BGFX_RESET_INTERNAL_FORCE is set (external callers don''t set this)

Where it matters

Apps/Playground/Android/.../BabylonNativeJNI.cpp surfaceChanged calls UpdateWindow + UpdateSize mid-render. Works only because Android-OpenGL re-binds. Same pattern on Win32 D3D11/D3D12, iOS Metal, or Linux X11 would silently keep rendering to the old window.

Options

1. Throw from UpdateWindow when initialized (symmetric with Throw from Device::UpdateDevice when rendering is enabled #1689). Callers do Disable → UpdateWindow → UpdateSize → Enable. Android JNI path needs updating; small efficiency regression vs EGL surface re-create.
2. Wait for uniform nwh re-bind in bgfx reset, then keep UpdateWindow reset-applicable everywhere. Depends on bgfx changes that don''t exist yet.