Correct usage of number of painting threads.

Previously this used the number of layout threads to allocate the
threadpool. This also makes the member name consistent with the rest of
the structure.

components/gfx/paint_task.rs

@@ -424,7 +424,7 @@ impl WorkerThreadProxy {
         let thread_count = if opts::get().gpu_painting {
             1
         } else {
-            opts::get().layout_threads
+            opts::get().paint_threads
         };
         (0..thread_count).map(|_| {
             let (from_worker_sender, from_worker_receiver) = channel();