Rendering to a surface and transparency

[waywardmonkeys]

It would be nice if the render_to_surface function would respect a transparent base_color and then show whatever is already rendered to the surface.

The main issue here is that BlendState::ALPHA_BLENDING would need to be set in the ColorTargetState in the BlitPipeline, but that's set for the Renderer and the background color is set in the RenderParams which is passed at each invocation of render_to_surface.

This also demonstrates that Renderer should perhaps be two different types, with a SurfaceRendener wrapping Renderer and storing the BlitPipeline and TargetTexture members, since they aren't used when rendering to a texture.

There are other issues with the current code, like if you're rendering to multiple surfaces and they're different sizes, then you'd be constantly creating and re-creating the TargetTexture.

Alternatively, maybe the render_to_surface should just be somewhere else and not part of the core library ...

[waywardmonkeys]

This would also impact using xilem in the current form if you have other content behind the UI (like a 3D application or game).