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/x11_platform.h

@@ -211,6 +211,8 @@ typedef struct _GLFWlibraryX11
     int             screen;
     Window          root;
 
+    // System content scale
+    float           contentScaleX, contentScaleY;
     // Helper window for IPC
     Window          helperWindowHandle;
     // Invisible cursor for hidden cursor mode