[macOS]: Call layer.setNeedsDisplay on show.

[grokys]

What does the pull request do?

The AvaloniaNative.GlPlatformSurface.CreateGlRenderTarget method can only be called on the UI thread. In normal circumstances this method is called in response to a call to TopLevel.HandlePaint from the native AvnView.updateLayer method. The flow of operations goes like this:

1. Window is created
2. Window is shown
3. Window is registered with the compositor
4. AvnView.updateLayer is called
5. Which calls TopLevel.HandlePaint
6. Which calls MediaContext.ImmediateRenderRequested to render on the UI thread
7. This render operation calls GlPlatformSurface.CreateGlRenderTarget on the UI thread and a GL render target is created
8. Subsequent render operations on the render thread use this render target

However, a customer was experiencing a problem where AvnView.updateLayer and by extension TopLevel.HandlePaint were being called before the window is shown. This broke the creation of the GL render target:

1. Window is created
2. AvnView.updateLayer is called
3. Which calls TopLevel.HandlePaint
4. Which calls MediaContext.ImmediateRenderRequested
5. But the window hasn't been registered with the compositor so nothing happens
6. Subsequent render operations are run on the render thread and try to create a render target
7. But this fails because the render target can only be created on the UI thread

This was happening because the customer is creating/showing the window by pushing jobs to the dispatcher, causing the HandlePaint request to come in earlier than expected.

What is the current behavior?

The window is blank until a resize happens (which causes HandlePaint to be called).

How was the solution implemented (if it's not obvious)?

The layer is invalidated at the end of a show operation, so AvnView.updateLayer/HandlePaint is called, creating the render target

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051999-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]