Internalise FrameBuffer and expose via IFrameBuffer

[smoogipoo]

Prereqs:

• Add IRenderer and adjust DrawNode signature #5324

• Adds the IFrameBuffer interface.

• Internalises FrameBuffer and adds a method to create frame buffers via IRenderer.

• Changes parameters of frame buffer creation to avoid ES30 references.

vNext

FrameBuffer is no longer exposed

Frame buffers may be created via the IRenderer and stored as an IFrameBuffer. Be aware that the parameter types have also changed!

class MyDrawNode : DrawNode
{
-   private readonly FrameBuffer myFrameBuffer = new FrameBuffer(new[] { RenderbufferInternalFormat.DepthComponent16 }, All.Nearest);
+   private IFrameBuffer myFrameBuffer;

    public override void Draw(IRenderer renderer)
    {
        base.Draw(renderer);

+       myFrameBuffer ??= renderer.CreateFrameBuffer(new[] { RenderBufferFormat.D16 }, TextureFilteringMode.Nearest);
    }

    protected override void Dispose(bool disposing)
    {
        base.Dispose(disposing);
        myFrameBuffer?.Dispose();
    }
}

Only a 16-bit depth format is supported for the time being, but more formats can be provided upon request.

[peppy]

Applied a bit of conflict resolution, but I don't think I broke anything. Changes look sane.