Add SSBO and compute shader support to gl wrappers

gl/query.h

@@ -1,5 +1,8 @@
 #pragma once
 
+#include <array>
+#include <optional>
+
 #include "object.h"
 
 namespace gl
@@ -33,4 +36,33 @@ public:
 
 	std::optional<int64_t> result();
 };
+
+// gpu time of a section of commands, from a pair of GL_TIMESTAMP queries. unlike GL_TIME_ELAPSED, timestamps
+// can be taken while another time query runs (e.g. the whole-frame query of the renderer). results are read
+// a few frames later without stalling; if the gpu falls further behind, the measurement is skipped
+class gpu_timer
+{
+  public:
+	gpu_timer() = default;
+	~gpu_timer();
+	gpu_timer(const gpu_timer &) = delete;
+	gpu_timer &operator=(const gpu_timer &) = delete;
+
+	void begin();
+	void end();
+	// duration of the most recent finished measurement, in nanoseconds
+	std::optional<uint64_t> result();
+
+  private:
+	void collect();
+
+	static constexpr size_t SLOTS = 4;
+	// start and end timestamp per slot, created on first use
+	std::array<GLuint, SLOTS * 2> m_queries{};
+	std::array<bool, SLOTS> m_pending{};
+	// slot written by the next begin(), also the oldest one still pending
+	size_t m_slot = 0;
+	bool m_active = false;
+	std::optional<uint64_t> m_last;
+};
 }