Add a second multi-thread method

method 1 grouping: [parse + reconstruction] // [filtering + output]
method 2 grouping: [parse] // [reconstruction+filtering + output]

Depending on some heuristics (see VP8ThreadMethod()), we
can pick one of the other when -mt flag (or option.use_threads)
is selected.

Conservatively, we always use method #2 for now until the heuristic
is refined (so, timing should be the same the before this patch)

+ replace 'use_threads' by 'mt_method'
+ define MIN_WIDTH_FOR_THREADS constant
+ fix comment alignment

Change-Id: I11a756dea9070d6e21b1a9481d357a1e8aa0663e

src/dec/vp8.c

@@ -662,8 +662,8 @@ static int ParseFrame(VP8Decoder* const dec, VP8Io* io) {
       return VP8SetError(dec, VP8_STATUS_USER_ABORT, "Output aborted.");
     }
   }
-  if (dec->use_threads_ && !WebPWorkerSync(&dec->worker_)) {
-    return 0;
+  if (dec->mt_method_ > 0) {
+    if (!WebPWorkerSync(&dec->worker_)) return 0;
   }
 
   // Finish
@@ -728,7 +728,7 @@ void VP8Clear(VP8Decoder* const dec) {
   if (dec == NULL) {
     return;
   }
-  if (dec->use_threads_) {
+  if (dec->mt_method_ > 0) {
     WebPWorkerEnd(&dec->worker_);
   }
   ALPHDelete(dec->alph_dec_);