Change some more GMs to clear to opaque black, not transparent black.

N.B.: this will change results on the bots for all the tests modified.

BUG=skia:3319

Review URL: https://codereview.chromium.org/1077763002
diff --git a/gm/displacement.cpp b/gm/displacement.cpp
index 91405d6..7b3d39c 100644
--- a/gm/displacement.cpp
+++ b/gm/displacement.cpp
@@ -68,7 +68,7 @@
             make_bitmaps();
             fInitialized = true;
         }
-        canvas->clear(0x00000000);
+        canvas->clear(SK_ColorBLACK);
         SkPaint paint;
         SkAutoTUnref<SkImageFilter> displ(SkBitmapSource::Create(fCheckerboard));
         paint.setImageFilter(SkDisplacementMapEffect::Create(