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/mir_monitor.c

@@ -88,6 +88,15 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
         *ypos = monitor->mir.y;
 }
 
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
+                                         float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = 1.f;
+    if (yscale)
+        *yscale = 1.f;
+}
+
 static void FillInRGBBitsFromPixelFormat(GLFWvidmode* mode, const MirPixelFormat pf)
 {
     switch (pf)