Fix `screen_get_dpi` on macOS for non fractional display scales and restore documentation.

doc/classes/DisplayServer.xml

@@ -521,6 +521,18 @@
 			<argument index="0" name="screen" type="int" default="-1">
 			</argument>
 			<description>
+				Returns the dots per inch density of the specified screen. If [code]screen[/code] is [/code]SCREEN_OF_MAIN_WINDOW[/code] (the default value), a screen with the main window will be used.
+				[b]Note:[/b] On macOS, returned value is inaccurate if fractional display scaling mode is used.
+				[b]Note:[/b] On Android devices, the actual screen densities are grouped into six generalized densities:
+				[codeblock]
+				   ldpi - 120 dpi
+				   mdpi - 160 dpi
+				   hdpi - 240 dpi
+				  xhdpi - 320 dpi
+				 xxhdpi - 480 dpi
+				xxxhdpi - 640 dpi
+				[/codeblock]
+				[b]Note:[/b] This method is implemented on Android, Linux, macOS and Windows. Returns [code]72[/code] on unsupported platforms.
 			</description>
 		</method>
 		<method name="screen_get_max_scale" qualifiers="const">

platform/osx/display_server_osx.mm

@@ -2246,11 +2246,18 @@ int DisplayServerOSX::screen_get_dpi(int p_screen) const {
 	NSArray *screenArray = [NSScreen screens];
 	if ((NSUInteger)p_screen < [screenArray count]) {
 		NSDictionary *description = [[screenArray objectAtIndex:p_screen] deviceDescription];
-		NSSize displayDPI = [[description objectForKey:NSDeviceResolution] sizeValue];
-		return (displayDPI.width + displayDPI.height) / 2;
+
+		const NSSize displayPixelSize = [[description objectForKey:NSDeviceSize] sizeValue];
+		const CGSize displayPhysicalSize = CGDisplayScreenSize([[description objectForKey:@"NSScreenNumber"] unsignedIntValue]);
+		float scale = [[screenArray objectAtIndex:p_screen] backingScaleFactor];
+
+		float den2 = (displayPhysicalSize.width / 25.4f) * (displayPhysicalSize.width / 25.4f) + (displayPhysicalSize.height / 25.4f) * (displayPhysicalSize.height / 25.4f);
+		if (den2 > 0.0f) {
+			return ceil(sqrt(displayPixelSize.width * displayPixelSize.width + displayPixelSize.height * displayPixelSize.height) / sqrt(den2) * scale);
+		}
 	}
 
-	return 96;
+	return 72;
 }
 
 float DisplayServerOSX::screen_get_scale(int p_screen) const {