Backends: GLFW: added a Win32-specific implementation of ImGui_ImplGlfw_GetContentScaleXXXX functions for legacy GLFW 3.2. (#9003)

Since we ship GLFW 3.2 binaries for Windows this makes our examples a little better behaving by default.
Ideally we'd use ImGui_ImplWin32_GetDpiScaleForHwnd() and ImGui_ImplWin32_GetDpiScaleForMonitor() but they are too much code to copy.