Implement emscripten backend + opengl3 example

[Teselka]

This is an implementation of the emscripten backend with opengl3 renderer as an example.

Features:

• Clipboard (without IME).
• Mouse (including mouse/touch/pen with IMGUI_IMPL_EMSCRIPTEN_POINTER_EVENTS, otherwise supports mouse only, touch input api worked bad for me).
• Keyboard (including edge case with tab/backspace interaction, see here).
• Mouse cursor and shape (implemented with document.querySelector(target_id).style.cursor = shape;)
• Dynamic canvas resizing (with user ImGui_ImplEmscripten_UpdateCanvasSize api).
• Gamepad.
• Numpad (i don't have keyboard with a numpad).

#8178

[Teselka]

Added gamepad support for it, tried to match final analog input with the win32 backend.