Draw the recorded instance draws with the variant seed they were recorded with

gl/query.h

@@ -2,6 +2,7 @@
 
 #include <array>
 #include <optional>
+#include <vector>
 
 #include "object.h"
 
@@ -65,4 +66,60 @@ class gpu_timer
 	bool m_active = false;
 	std::optional<uint64_t> m_last;
 };
+
+// gpu time of the parts of a frame, from GL_TIMESTAMP queries put one after another: a part lasts from its mark to the next
+// one, so the parts add up to the time from the first mark to the last, with nothing left out and nothing counted twice.
+// the first mark of a gpu_timer is put while the gpu may still be busy with what came before, so its time can hold the tail
+// of that work; in the run of marks that time goes to the part it belongs to. results are read a few frames later without
+// stalling; if the gpu falls further behind, the frame is skipped
+class gpu_timeline
+{
+  public:
+	struct part
+	{
+		char const *name;
+		uint64_t time; // nanoseconds
+	};
+
+	gpu_timeline() = default;
+	~gpu_timeline();
+	gpu_timeline(const gpu_timeline &) = delete;
+	gpu_timeline &operator=(const gpu_timeline &) = delete;
+
+	// starts the frame with its first part. the names are kept as pointers: string literals
+	void begin(char const *Name);
+	// ends the part under way and starts the next one
+	void next(char const *Name);
+	void end();
+	bool active() const
+	{
+		return m_active;
+	}
+	// parts of the most recent finished frame in the order they came first, the time of parts of the same name added up
+	std::vector<part> const &result();
+	// from the first mark of that frame to the last
+	uint64_t total() const
+	{
+		return m_total;
+	}
+
+  private:
+	void collect();
+
+	static constexpr size_t SLOTS = 4;
+	static constexpr size_t MARKS = 32; // per frame, the last one included
+	struct frame
+	{
+		std::array<GLuint, MARKS> queries{};
+		std::array<char const *, MARKS> names{};
+		size_t count = 0; // marks put
+		bool pending = false;
+	};
+	std::array<frame, SLOTS> m_frames;
+	// slot written by the next begin(), also the oldest one still pending
+	size_t m_slot = 0;
+	bool m_active = false;
+	std::vector<part> m_last;
+	uint64_t m_total = 0;
+};
 }