Each window created in imgui_impl_win32.cpp now creates an openGL
context which shares its display list space with the primary window's.
IMGUI_IMPL_WIN32_OPENGL3 must be defined to use this fix.

ImGui_ImplWin32_Init() now optionally accepts a render context handle,
and IMGUI_IMPL_WIN32_OPENGL3 is no longer a necessary define.

Device Contexts are no longer maintained for the application lifetime.
::GetDC() is now never called when using DirectX backend.
Removed lambda callback and brought WGL function loading into
conformance with XInput DLL loading.