GUI: Don't assume ManagedSurface's pitch

This caused corruption with small scale's cursor (18px) if
ManagedSurface's inner surface used a different pitch than w *
format.bytesPerPixel.

gui/ThemeEngine.cpp

@@ -1639,6 +1639,8 @@ bool ThemeEngine::createCursor(const Common::String &filename, int hotspotX, int
 			const int index = colorToIndex[col];
 			_cursor[y * _cursorWidth + x] = index;
 		}
+
+		src += cursor->pitch - cursor->w * cursor->format.bytesPerPixel;
 	}
 
 	_useCursor = true;