Add: [OSX] Automatic zoom level suggestion for Cocoa video driver.

src/video/cocoa/cocoa_v.h

@@ -73,6 +73,7 @@ public:
 
 protected:
 	Dimension GetScreenSize() const override;
+	float GetDPIScale() override;
 
 private:
 	bool PollEvent();

src/video/cocoa/cocoa_v.mm

@@ -294,6 +294,12 @@ Dimension VideoDriver_Cocoa::GetScreenSize() const
 	return { static_cast<uint>(NSWidth(frame)), static_cast<uint>(NSHeight(frame)) };
 }
 
+/** Get DPI scale of our window. */
+float VideoDriver_Cocoa::GetDPIScale()
+{
+	return this->cocoaview != nil ? [ this->cocoaview getContentsScale ] : 1.0f;
+}
+
 /**
  * Are we in fullscreen mode?
  * @return whether fullscreen mode is currently used