twn_draw.c: add draw_quad()

include/twn_draw.h

@@ -55,6 +55,14 @@ TWN_API void draw_triangle(char const *texture,
                            Color       c1,      /* optional, default: all 255 */
                            Color       c2);     /* optional, default: all 255 */
 
+TWN_API void draw_quad(char const *texture,
+                       Vec3        v0,          /* upper-left */
+                       Vec3        v1,          /* bottom-left */
+                       Vec3        v2,          /* bottom-right */
+                       Vec3        v3,          /* upper-right */
+                       Rect        texture_region,
+                       Color       color);      /* optional, default: all 255 */
+
 TWN_API void draw_billboard(const char *texture,
                             Vec3 position,
                             Vec2 size,