Draw the whole 3D view in place of the client's renderer

The full view is now the default. While the renderer process covers the whole
view (Channel::covering, set after each frame it presents), the adapter skips
the client's renderer's clears and draws, decided per frame; the client's
renderer still gets state and resources, and draws again as soon as the
renderer process stops covering the view.

The client's window no longer gets WS_CLIPCHILDREN: the client draws when its
window gets WM_PAINT, which a window entirely covered by clipped children never
gets (it fell to about one frame a second). The flip model swap chain shows
above the client's drawing anyway.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

adapter/renderer_process.hpp

@@ -27,6 +27,9 @@ public:
 
     [[nodiscard]] bool alive() const { return alive_; }
 
+    // Whether the renderer process covers the whole 3D view (protocol::Channel::covering).
+    [[nodiscard]] bool covering() const { return alive_ && channel_->covering != 0; }
+
 private:
     RendererProcess() = default;