HiDPI support

[Stehfyn]

Windows 10-post some build tries to preempt non-dpi-aware scaling issues for processes on HiDPI monitors, which results in pre-scaled values returned by various win32 functions which are otherwise unexpected by imgui. The solution is relatively simple, but is otherwise not solvable by simply using ImGui_ImplWin32_EnableDpiAwareness(), as on builds Windows10 or higher that function will use SetThreadDpiAwarenessContext() as opposed to SetProcessDpiAwareness()

void ImGui_ImplWin32_EnableDpiAwareness()
{
    // Make sure monitors will be updated with latest correct scaling
    if (ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData())
        bd->WantUpdateMonitors = true;


    if (_IsWindows10OrGreater())
    {
        static HINSTANCE user32_dll = ::LoadLibraryA("user32.dll"); // Reference counted per-process
        if (PFN_SetThreadDpiAwarenessContext SetThreadDpiAwarenessContextFn = (PFN_SetThreadDpiAwarenessContext)::GetProcAddress(user32_dll, "SetThreadDpiAwarenessContext"))
        {
            SetThreadDpiAwarenessContextFn(DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2);
            return;
        }
    }
    if (_IsWindows8Point1OrGreater())
    {
        static HINSTANCE shcore_dll = ::LoadLibraryA("shcore.dll"); // Reference counted per-process
        if (PFN_SetProcessDpiAwareness SetProcessDpiAwarenessFn = (PFN_SetProcessDpiAwareness)::GetProcAddress(shcore_dll, "SetProcessDpiAwareness"))
        {
            SetProcessDpiAwarenessFn(PROCESS_PER_MONITOR_DPI_AWARE);
            return;
        }
    }
#if _WIN32_WINNT >= 0x0600
    ::SetProcessDPIAware();
#endif
}

https://github.com/ocornut/imgui/blob/dad1047b04e335454d17cd546c29a44173da8eb8/backends/imgui_impl_win32.cpp#L930-L957

Using a combination of both SetProcessDpiAwareness() and ImGui_ImplWin32_EnableDpiAwareness() can do the trick, and disable Windows' preemptive scaling for HiDPI monitors.