funnel all constant colors through append_constant_color()

My next step is to change the uniform_color context to

    struct {
       float r,g,b,a;
       uint32_t rgba;
    };

so that it's trivial to load in both float and 8-bit pipelines.

Change-Id: If9bdde353ced3bf9eb0c63204b4770ed614ad16b
Reviewed-on: https://skia-review.googlesource.com/30481
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/tests/SRGBTest.cpp b/tests/SRGBTest.cpp
index 1db9eb3..cda6fc4 100644
--- a/tests/SRGBTest.cpp
+++ b/tests/SRGBTest.cpp
@@ -71,8 +71,9 @@
 
     SkJumper_MemoryCtx dst = { &color, 0 };
 
-    SkRasterPipeline_<256> p;
-    p.append(SkRasterPipeline::uniform_color, &color);
+    SkSTArenaAlloc<256> alloc;
+    SkRasterPipeline p(&alloc);
+    p.append_constant_color(&alloc, color);
     p.append(SkRasterPipeline::to_srgb);
     p.append(SkRasterPipeline::store_f32, &dst);
     p.run(0,0,4,1);