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.

include/GLFW/glfw3.h

@@ -973,6 +973,10 @@ extern "C" {
  *  [attribute](@ref GLFW_CLIENT_API_attrib).
  */
 #define GLFW_CONTEXT_CREATION_API   0x0002200B
+/*! @brief Window content area scaling window
+ *  [window hint](@ref GLFW_SCALE_TO_MONITOR).
+ */
+#define GLFW_SCALE_TO_MONITOR       0x0002200C
 
 #define GLFW_COCOA_RETINA_FRAMEBUFFER 0x00023001
 #define GLFW_COCOA_FRAME_NAME         0x00023002