Replace buggy_blend_modes GM with an exhaustive test.

include/core/SkColorPriv.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkColorPriv_DEFINED
 #define SkColorPriv_DEFINED
 
@@ skipping 351 matching lines @@
             uint32_t alpha_color_value = SkGetPackedA32(pm_color_value);    \
             SkA32Assert(alpha_color_value);                                 \
             SkASSERT(SkGetPackedR32(pm_color_value) <= alpha_color_value);  \
             SkASSERT(SkGetPackedG32(pm_color_value) <= alpha_color_value);  \
             SkASSERT(SkGetPackedB32(pm_color_value) <= alpha_color_value);  \
         } while (false)
 #else
     #define SkPMColorAssert(c)
 #endif
 
+static inline bool SkPMColorValid(SkPMColor c) {
+    auto a = SkGetPackedA32(c);
+    bool valid = a <= SK_A32_MASK
+              && SkGetPackedR32(c) <= a
+              && SkGetPackedG32(c) <= a
+              && SkGetPackedB32(c) <= a;
+    if (valid) {
+        SkPMColorAssert(c);  // Make sure we're consistent when it counts.
+    }
+    return valid;
+}
+
 /**
  *  Pack the components into a SkPMColor, checking (in the debug version) that
  *  the components are 0..255, and are already premultiplied (i.e. alpha >= color)
  */
 static inline SkPMColor SkPackARGB32(U8CPU a, U8CPU r, U8CPU g, U8CPU b) {
     SkA32Assert(a);
     SkASSERT(r <= a);
     SkASSERT(g <= a);
     SkASSERT(b <= a);
 
@@ skipping 649 matching lines @@
     int srcG = SkColorGetG(src);
     int srcB = SkColorGetB(src);
 
     for (int i = 0; i < width; i++) {
         dst[i] = SkBlendLCD16Opaque(srcR, srcG, srcB, dst[i], mask[i],
                                     opaqueDst);
     }
 }
 
 #endif