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.

src/win32_platform.h

@@ -306,6 +306,7 @@ typedef struct _GLFWwindowWin32
     GLFWbool            maximized;
     // Whether to enable framebuffer transparency on DWM
     GLFWbool            transparent;
+    GLFWbool            scaleToMonitor;
 
     // The last received cursor position, regardless of source
     int                 lastCursorPosX, lastCursorPosY;