Draw render targets in the renderer process

While the renderer process runs, the adapter makes the client's render targets
itself and the renderer process draws frames into them: texture targets are
drawn with like any texture, and a pixmap target's frame is read back through
the channel into its Pixmap (protocol version 4: BeginFrame names its target,
CreateRenderTarget, Channel::readSize and the -read event). Frames into render
targets are drawn even while the window is hidden.

The client uses pixmap targets for snapshots (outfit pictures, photos), checked
to look right. The render target interfaces are declared from IMVU's vtables.

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

adapter/renderer_process.hpp

@@ -25,6 +25,10 @@ public:
     // process is gone or stopped taking them (it's then given up on).
     bool send(std::span<const std::byte> commands, bool endsFrame);
 
+    // After sending a frame that reads its render target back: waits for the pixels (protocol::Channel), valid until
+    // the next send. Empty if the renderer process couldn't, or is gone.
+    std::span<const std::byte> readBack();
+
     [[nodiscard]] bool alive() const { return alive_; }
 
     // Whether the renderer process covers the whole 3D view (protocol::Channel::covering).
@@ -36,6 +40,7 @@ private:
     HANDLE mapping_ = nullptr;
     HANDLE ready_ = nullptr;
     HANDLE consumed_ = nullptr;
+    HANDLE read_ = nullptr;
     HANDLE process_ = nullptr;
     protocol::Channel* channel_ = nullptr;
     std::byte* data_ = nullptr;