try not zeroing registers in start_pipeline

Generally stages take care of state setup themselves, either with
seed_shader, constant_color, a load, etc.  I think these zeros may
be unnecessarily cautious.

This can't make anything draw more correctly, but it could make things
   - draw wrong
   - draw more slowly
   - draw more quickly
so it's an interesting thing to try and keep an eye on.

Change-Id: I7e5ea3cd79e55a65e1dbd214601e147ba3815b87
Reviewed-on: https://skia-review.googlesource.com/20976
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/jumper/SkJumper_stages.cpp

@@ -53,7 +53,11 @@ using Stage = void(K* k, void** program, size_t x, size_t y, size_t tail, F,F,F,
 #endif
 MAYBE_MSABI
 extern "C" void WRAP(start_pipeline)(size_t x, size_t y, size_t limit, void** program, K* k) {
+#if defined(JUMPER)
+    F v;
+#else
     F v{};
+#endif
     auto start = (Stage*)load_and_inc(program);
     while (x + kStride <= limit) {
         start(k,program,x,y,0,    v,v,v,v, v,v,v,v);

src/jumper/SkJumper_stages_lowp.cpp

@@ -71,7 +71,11 @@ using Stage = void(K* k, void** program, size_t x, size_t y, size_t tail, F,F,F,
 #endif
 MAYBE_MSABI
 extern "C" size_t WRAP(start_pipeline)(size_t x, size_t y, size_t limit, void** program, K* k) {
+#if defined(JUMPER)
+    F v;
+#else
     F v{};
+#endif
     auto start = (Stage*)load_and_inc(program);
     while (x + kStride <= limit) {
         start(k,program,x,y,0,    v,v,v,v, v,v,v,v);