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/null_monitor.c

@@ -36,6 +36,15 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
 {
 }
 
+void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
+                                         float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = 1.f;
+    if (yscale)
+        *yscale = 1.f;
+}
+
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* found)
 {
     return NULL;