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

src/rendering/twn_gl_any_rendering.c

@@ -26,7 +26,7 @@ void delete_vertex_buffer(VertexBuffer buffer) {
 }
 
 
-void specify_vertex_buffer(VertexBuffer buffer, void *data, size_t bytes) {
+void specify_vertex_buffer(VertexBuffer buffer, void const *data, size_t bytes) {
     glBindBuffer(GL_ARRAY_BUFFER, buffer);
     glBufferData(GL_ARRAY_BUFFER, bytes, data, GL_STREAM_DRAW);
 }