use pbo for picking buffers, restore picking for scenery nodes

gl/pbo.h

@@ -0,0 +1,17 @@
+#include "buffer.h"
+#include "fence.h"
+#include <optional>
+
+namespace gl {
+    class pbo : private buffer
+    {
+        std::optional<fence> sync;
+        int size = 0;
+        bool data_ready;
+
+    public:
+        void request_read(int x, int y, int lx, int ly);
+        bool read_data(int lx, int ly, uint8_t *data);
+        bool is_busy();
+    };
+}