Throw an error when cursor offset is oob on SDL (#733)

Fix custom default cursor not restoring on DX
Add SDL note in cursor sample.
Improve text visibility in cursor sample.

hxd/System.hl.hx

@@ -216,6 +216,9 @@ class System {
 					var pixels = frame.getPixels();
 					pixels.convert(BGRA);
 					#if hlsdl
+					if (c.offsetX < 0 || c.offsetX >= pixels.width || c.offsetY < 0 || c.offsetY >= pixels.height) {
+						throw "SDL2 does not allow creation of cursors with offset outside of cursor image bounds.";
+					}
 					var surf = sdl.Surface.fromBGRA(pixels.bytes, pixels.width, pixels.height);
 					c.alloc.push(sdl.Cursor.create(surf, c.offsetX, c.offsetY));
 					surf.free();

hxd/Window.hl.hx

@@ -171,8 +171,10 @@ class Window {
 				#end
 			case Enter:
 				#if hldx
-				// reset cursor to default
-				dx.Cursor.createSystem(Arrow).set();
+				// Restore cursor
+				var cur = @:privateAccess hxd.System.currentNativeCursor;
+				@:privateAccess hxd.System.currentNativeCursor = null;
+				hxd.System.setNativeCursor(cur);
 				#end
 				event(new Event(EOver));
 			case Leave:

samples/Cursor.hx

@@ -36,6 +36,7 @@ class Cursor extends SampleApp {
 			var i = new h2d.Interactive(120, 20, s2d);
 			var tf = new h2d.Text(hxd.res.DefaultFont.get(), i);
 			tf.text = c.getName();
+			tf.dropShadow = { dx: 1, dy: 1, color: 0, alpha: 1 };
 			tf.x = 5;
 			i.x = 0;
 			i.y = pos++ * 20;
@@ -63,6 +64,9 @@ class Cursor extends SampleApp {
 		// `hxd.System.setCursor` function.
 		// Useful when game utilizes stylized cursors for everything.
 
+		// HLSDL note: Cursor offsetX and offsetY should remain inside frame bounds.
+		// This is a limitation of SDL (most likely for portability reasons).
+
 		var doOverride = false;
 		var defOverride = new hxd.BitmapData(10, 10);
 		defOverride.line(0, 0, 5, 0, 0xffff0000);