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.cpp

@@ -69,6 +69,18 @@ void Recorder::addPixels(protocol::Op op, std::uint32_t id, const imvu::Pixmap&
     flushIfLarge();
 }
 
+std::uint32_t Recorder::createRenderTarget(std::uint32_t width, std::uint32_t height)
+{
+    const std::uint32_t id = nextId_++;
+    add(protocol::Op::CreateRenderTarget, protocol::CreateRenderTarget{id, width, height});
+    return id;
+}
+
+std::span<const std::byte> Recorder::readBack()
+{
+    return active() ? process_->readBack() : std::span<const std::byte>{};
+}
+
 std::uint32_t Recorder::createTexture(const imvu::Pixmap& pixmap, std::uint32_t flags)
 {
     const std::uint32_t id = nextId_++;