rendering: use sprite batching techniques for rect primitives, unite their render path

include/twn_texture_modes.h

@@ -1,6 +1,9 @@
 #ifndef TWN_TEXTURES_MODES_H
 #define TWN_TEXTURES_MODES_H
 
+/* TODO: rename, as it doesn't have to be about textures only, but blending */
+/* TODO: move from public /include/ tree */
+
 /* alpha channel information */
 typedef enum TextureMode {
     TEXTURE_MODE_OPAQUE,        /* all pixels are solid */