OpenGLControlBase: allow changing the rendering order of the avalonia layer and the opengl layer

[vpenades]

Is your feature request related to a problem? Please describe.

OpenGLControlBase has two rendering stages executed in the following order:

1. Avalonia's own Render method
2. OnOpenGlRender

This is probably useful in some scenarios where you want to overlay openGL on top of avalonia controls.

The problem I have is that I am working on a CAD application where I need to render some 2D overlays and toolings on top of openGL, and it makes sense to do so using Avalonia instead of GL

But right now, to do so, I need to create an extra canvas control on top of the OpenGLControlBase, where I could simply use OpenGLControlBase itself, if the Avalonia's rendering happens after GL rendering.

Describe the solution you'd like

I would like to have a boolean property in OpenGLControlBase to indicate the Avalonia's and OpenGL rendering order, so when set, Avalonia's Render is drawn on top of OnOpenGlRender draw.

Describe alternatives you've considered

Right now I am creating an additional Canvas control on top of OpenGlControlBase

The drawback is, being two controls, the wiring is a lot more complex because what I am redering in both layers must match

Additional context

No response