Fix array overrun and add test.

tests/BlitMaskClip.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkBlitter.h"
+#include "SkMask.h"
+#include "Test.h"
+
+class TestBlitter : public SkBlitter {
+public:
+    TestBlitter(SkIRect bounds, skiatest::Reporter* reporter)
+        : fBounds(bounds)
+        , fReporter(reporter) { }
+
+    void blitH(int x, int y, int width) override {
+
+        REPORTER_ASSERT(fReporter, x >= fBounds.fLeft && x < fBounds.fRight);
+        REPORTER_ASSERT(fReporter, y >= fBounds.fTop && y < fBounds.fBottom);
+        int right = x + width;
+        REPORTER_ASSERT(fReporter, right > fBounds.fLeft && right <= fBounds.fRight);
+    }
+
+private:
+    SkIRect fBounds;
+    skiatest::Reporter* fReporter;
+};
+
+
+// Exercise all clips compared with different widths of bitMask. Make sure that no buffer
+// overruns happen.
+DEF_TEST(BlitAndClip, reporter) {
+    const int originX = 100;
+    const int originY = 100;
+    for (int width = 1; width <= 32; width++) {
+        const int height = 2;
+        int rowBytes = (width + 7) >> 3;
+        uint8_t* bits = new uint8_t[rowBytes * height];
+
+        SkIRect b = {originX, originY, originX + width, originY + height};
+
+        SkMask mask;
+        mask.fFormat = SkMask::kBW_Format;
+        mask.fBounds = b;
+        mask.fImage = (uint8_t*)bits;
+        mask.fRowBytes = rowBytes;
+
+        TestBlitter tb(mask.fBounds, reporter);
+
+        for (int top = b.fTop; top < b.fBottom; top++) {
+            for (int bottom = top + 1; bottom <= b.fBottom; bottom++) {
+                for (int left = b.fLeft; left < b.fRight; left++) {
+                    for (int right = left + 1; right <= b.fRight; right++) {
+                        SkIRect clipRect = {left, top, right, bottom};
+                        tb.blitMask(mask, clipRect);
+                    }
+                }
+            }
+        }
+
+        delete [] bits;
+    }
+}