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/recorder.hpp

@@ -5,6 +5,7 @@
 
 #include <cstdint>
 #include <memory>
+#include <span>
 
 #include "renderer_process.hpp"
 #include "t5de/imvu/types.hpp"
@@ -37,6 +38,10 @@ public:
     void flushIfLarge();
 
     std::uint32_t createTexture(const imvu::Pixmap& pixmap, std::uint32_t flags);
+    // Destroyed with destroyTexture.
+    std::uint32_t createRenderTarget(std::uint32_t width, std::uint32_t height);
+    // After flushing a frame that reads its render target back: its pixels (RendererProcess::readBack).
+    std::span<const std::byte> readBack();
     void updateTexture(std::uint32_t id, const imvu::Pixmap& pixmap);
     void destroyTexture(std::uint32_t id);