added camera pan for parallax and improved line drawing with preview line

engine/shape.h

@@ -47,6 +47,20 @@ protected:
     }
 };
 
+class LineSegment : public Shape
+{
+    void create_impl(GLushort* idx, vertex_t* vertices);
+public:
+    bool create()
+    {
+        static GLushort idx[2];
+        static vertex_t vertices[2];
+        create_impl(idx, vertices);
+        return create_buffers(idx, vertices, sizeof(idx), sizeof(vertices));
+    }
+    void update_vertices(const glm::vec4 data[2]);
+};
+
 class Plane : public Shape
 {
     void create_impl(float w, float h, int div, GLushort* idx, vertex_t* vertices);