add orbit helper function in main.cpp

src/main.cpp

@@ -110,7 +110,19 @@ loadScene(RenderState* rs)
 	return initLights(rs);
 }
 
-#include <glm/gtc/matrix_transform.hpp>
+void
+orbitPositionZ0(glm::vec4* pos, float angle)
+{
+	// get radius length
+	float r = sqrt(pow(abs(pos->x), 2) + pow(abs(pos->z), 2));
+	// get current angle about z axis
+	float a = atan2f(pos->z, pos->x);
+	// apply increment
+	a += angle;
+	// get new x/z components
+	pos->x = r * cos(a);
+	pos->z = r * sin(a);
+}
 
 void
 render_cb_pre(RenderState* rs)