separate the rest of general drawing code

include/twn_draw.h

@@ -68,7 +68,9 @@ TWN_API void draw_triangle(char const *texture,
 
 TWN_API void draw_billboard(const char *path,
                             Vec3 position,
-                            Vec2 size);
+                            Vec2 size,
+                            Color color,        /* optional, default: all 255 */
+                            bool cylindrical);  /* optional, default: false */
 
 /* sets a perspective 3d camera to be used for all 3d commands */
 TWN_API void draw_camera(Vec3 position, float fov, Vec3 up, Vec3 direction);

include/twn_types.h

@@ -49,7 +49,7 @@ typedef struct Rect {
     float h;
 } Rect;
 
-
+/* TODO: remove from here? */
 typedef struct Matrix4 {
     Vec4 row[4];
 } Matrix4;