Make SkBlitter hierarchy explicit about what needs to be implemented.

src/core/SkRegion_path.cpp

 /*
  * 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.
  */
 
 #include "SkRegionPriv.h"
 #include "SkBlitter.h"
 #include "SkScan.h"
@@ skipping 27 matching lines @@
                 fCurrScanline = fCurrScanline->nextScanline();
             }
         }
     }
 
     int     computeRunCount() const;
     void    copyToRect(SkIRect*) const;
     void    copyToRgn(SkRegion::RunType runs[]) const;
 
     void blitH(int x, int y, int width) override;
+    void blitAntiH(int x, int y, const SkAlpha antialias[], const int16_t runs[]) override {
+        SkDEBUGFAIL("blitAntiH not implemented");
+    }
 
 #ifdef SK_DEBUG
     void dump() const {
         SkDebugf("SkRgnBuilder: Top = %d\n", fTop);
         const Scanline* line = (Scanline*)fStorage;
         while (line < fCurrScanline) {
             SkDebugf("SkRgnBuilder::Scanline: LastY=%d, fXCount=%d", line->fLastY, line->fXCount);
             for (int i = 0; i < line->fXCount; i++) {
                 SkDebugf(" %d", line->firstX()[i]);
             }
@@ skipping 470 matching lines @@
 #endif
 
     path->incReserve(count << 1);
     do {
         SkASSERT(count > 1);
         count -= extract_path(start, stop, path);
     } while (count > 0);
 
     return true;
 }