fix mixing up of SDL and libc allocators, proper flushing of quad build buffers

src/rendering/twn_gl_any_rendering.c

@@ -31,6 +31,7 @@ void delete_vertex_buffer(VertexBuffer buffer) {
 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);
+    glBindBuffer(GL_ARRAY_BUFFER, 0);
 }