Support mapping gfx::buffer (write-only)

libs/gfx/include/psemek/gfx/buffer.hpp

@@ -3,6 +3,7 @@
 #include <psemek/gfx/gl.hpp>
 
 #include <vector>
+#include <memory>
 
 namespace psemek::gfx
 {
@@ -41,8 +42,24 @@ namespace psemek::gfx
 			load(data.data(), data.size(), usage);
 		}
 
+		template <typename T>
+		std::shared_ptr<T> map()
+		{
+			if (auto p = mapped_.lock())
+				return std::static_pointer_cast<T>(p);
+
+			bind();
+			std::shared_ptr<T> p(reinterpret_cast<T *>(gl::MapBuffer(gl::ARRAY_BUFFER, gl::WRITE_ONLY)), [this](T *){
+				bind();
+				gl::UnmapBuffer(gl::ARRAY_BUFFER);
+			});
+			mapped_ = p;
+			return p;
+		}
+
 	private:
 		GLuint id_;
+		std::weak_ptr<void> mapped_;
 
 		explicit buffer(std::nullptr_t);
 	};