Fix CefNet.Avalonia Compat

[AigioL]

What does the pull request do?

ca408e5 Breaking the compatibility
CefNet.Avalonia required cursor handle.
https://github.com/CefNet/CefNet/blob/master/CefNet.Avalonia/Internal/CursorInteropHelper.cs#L18

What is the current behavior?

System.TypeInitializationException
  HResult=0x80131534
  Message=The type initializer for 'CefNet.Internal.CursorInteropHelper' threw an exception.
  Source=CefNet.Avalonia
  StackTrace:
   在 CefNet.Internal.CursorInteropHelper.Create(IntPtr cursorHandle)
   在 CefNet.Internal.AvaloniaWebViewGlue.OnCursorChange(CefBrowser browser, IntPtr cursorHandle, CefCursorType type, CefCursorInfo customCursorInfo)
   在 CefNet.Internal.CefDisplayHandlerGlue.OnCursorChange(CefBrowser browser, IntPtr cursor, CefCursorType type, CefCursorInfo customCursorInfo)
   在 CefNet.CefDisplayHandler.OnCursorChangeImpl(cef_display_handler_t* self, cef_browser_t* browser, IntPtr cursor, CefCursorType type, cef_cursor_info_t* custom_cursor_info)

内部异常 1:
MissingMethodException: Method not found: 'Avalonia.Platform.IPlatformHandle Avalonia.Input.Cursor.get_PlatformCursor()'.

[dnfadmin]

All CLA requirements met.

[grokys]

@kekekeks are we OK exposing IntPtr handles in *Impl interfaces or should IPlatformHandle be used?

[Gillibald]

Looks like only the Win32 platform uses a handle and the rest sets the property to IntPtr.Zero so this change is wrong and redundant Handle properties should be removed.

On Win32 just cast to IPlatformHandle: https://github.com/AvaloniaUI/Avalonia/blob/master/src/Windows/Avalonia.Win32/CursorFactory.cs#L170

[AigioL]

看起来只有 Win32 平台使用句柄， 其余的设置属性， 所以这种变化是错误的， 多余的属性应该删除。IntPtr.Zero``Handle

在Win32刚刚投到：https://github.com/AvaloniaUI/Avalonia/blob/master/src/Windows/Avalonia.Win32/CursorFactory.cs#L170IPlatformHandle

It also seems to have influenced macOS and Linux

https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.X11/X11CursorFactory.cs#L72
https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.X11/X11CursorFactory.cs#L141
https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.X11/X11CursorFactory.cs#L88
https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.X11/X11CursorFactory.cs#L77

Mobile platforms may not need
Maybe should let the desktop implementation classes inherit IPlatformHandle
Or set up two interfaces to distinguish between mobile platform (IMobileCursorImpl) and desktop platform (IDesktopCursorImpl)

[kekekeks]

All platform handles should be exposed as IPlatformHandle. In some cases platform handles might be non-representable as a plain IntPtr without additional info. For Win32 we can use PlatformHandle class with HCURSOR as description value. For X11 it should be XCursor.