twn_draw.h: new camera api

src/twn_camera_c.h

@@ -0,0 +1,21 @@
+#ifndef TWN_CAMERA_H
+#define TWN_CAMERA_H
+
+#include "twn_types.h"
+
+/* TODO: make it cached? */
+/*       for example, perspective matrix only needs recaluclation on FOV change */
+
+/* first person camera class */
+typedef struct Camera {
+    Vec3 pos;        /* eye position */
+    Vec3 target;     /* normalized target vector */
+    Vec3 up;         /* normalized up vector */
+    float fov;       /* field of view, in radians */
+} Camera;
+
+Matrix4 camera_look_at(const Camera *camera);
+
+Matrix4 camera_perspective(const Camera *const camera);
+
+#endif