Add GLFW_SCALE_TO_MONITOR

This adds the GLFW_SCALE_TO_MONITOR window hint for automatically
resizing the content area of a window to the requested size times the
monitor content scale each time it is placed on a new monitor.  This
only applies to windowed mode windows and includes the initial placement
at window creation.

This hint only has an effect on platforms where screen coordinates and
pixels always map 1:1 such as Windows and X11.  Platforms like macOS
instead change the resolution of the framebuffer independently of the
window size.

Related to #676.
Related to #1115.

src/x11_window.c

@@ -592,6 +592,15 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
                                    const _GLFWwndconfig* wndconfig,
                                    Visual* visual, int depth)
 {
+    int width = wndconfig->width;
+    int height = wndconfig->height;
+
+    if (wndconfig->scaleToMonitor)
+    {
+        width *= _glfw.x11.contentScaleX;
+        height *= _glfw.x11.contentScaleY;
+    }
+
     // Create a colormap based on the visual used by the current context
     window->x11.colormap = XCreateColormap(_glfw.x11.display,
                                            _glfw.x11.root,
@@ -617,7 +626,7 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
         window->x11.handle = XCreateWindow(_glfw.x11.display,
                                            _glfw.x11.root,
                                            0, 0,
-                                           wndconfig->width, wndconfig->height,
+                                           width, height,
                                            0,      // Border width
                                            depth,  // Color depth
                                            InputOutput,
@@ -720,7 +729,7 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
         XFree(hints);
     }
 
-    updateNormalHints(window, wndconfig->width, wndconfig->height);
+    updateNormalHints(window, width, height);
 
     // Set ICCCM WM_CLASS property
     {