Add content scale queries

This adds glfwGetWindowContentScale and glfwGetMonitorContentScale for
querying the recommended drawing scale factor for DPI-aware rendering.

Parts of this patch are based on code by @ferreiradaselva.

Fixes #235.
Fixes #439.
Fixes #677.
Fixes #845.
Fixes #898.

src/win32_window.c

@@ -983,19 +983,6 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
             break;
         }
 
-        case WM_DPICHANGED:
-        {
-            RECT* rect = (RECT*) lParam;
-            SetWindowPos(window->win32.handle,
-                         HWND_TOP,
-                         rect->left,
-                         rect->top,
-                         rect->right - rect->left,
-                         rect->bottom - rect->top,
-                         SWP_NOACTIVATE | SWP_NOZORDER);
-            break;
-        }
-
         case WM_DROPFILES:
         {
             HDROP drop = (HDROP) wParam;
@@ -1415,6 +1402,14 @@ void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window,
         *bottom = rect.bottom - height;
 }
 
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window,
+                                        float* xscale, float* yscale)
+{
+    const HANDLE handle = MonitorFromWindow(window->win32.handle,
+                                            MONITOR_DEFAULTTONEAREST);
+    _glfwGetMonitorContentScaleWin32(handle, xscale, yscale);
+}
+
 void _glfwPlatformIconifyWindow(_GLFWwindow* window)
 {
     ShowWindow(window->win32.handle, SW_MINIMIZE);