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_window.c

@@ -515,6 +515,15 @@ void _glfwPlatformGetWindowSize(_GLFWwindow* window, int* width, int* height)
         *height = window->mir.height;
 }
 
+void _glfwPlatformGetWindowContentScale(_GLFWwindow* window,
+                                        float* xscale, float* yscale)
+{
+    if (xscale)
+        *xscale = 1.f;
+    if (yscale)
+        *yscale = 1.f;
+}
+
 void _glfwPlatformIconifyWindow(_GLFWwindow* window)
 {
     MirWindowSpec* spec;