Listen for browser paste events and directly inject clipboard text via
AddInputCharactersUTF8. Also suppress character input when Ctrl/Meta is
held to prevent 'v' from being typed on Cmd+V.
The Emscripten backend was missing Platform_SetClipboardTextFn/GetClipboardTextFn,
so ImGui::SetClipboardText was a no-op in the browser.
Hook navigator.clipboard.writeText for writes and keep a local copy for reads.
ImGui OpenGL loader is patched, and not regenerated, because the gl3w
script requires network connectivity to download the header files from
Khronos at build time.
