Fix rendering error in the sampler

This error would cause the sampler to render incomplete buffers
whenever multiple LV2 atoms were received in the run() stage.

plugin/plugin.c

@@ -248,7 +248,9 @@ static void handle_event(struct sampler_plugin *plugin, LV2_Atom_Event *ev) {
 
 static void render(struct sampler_plugin *plugin, uint32_t start,
                    uint32_t end) {
-  sampler_output(plugin->sampler, plugin->output_port_l, plugin->output_port_r,
+  sampler_output(plugin->sampler,
+                 plugin->output_port_l + start,
+                 plugin->output_port_r + start,
                  end - start);
 }