add isRect() check to AAClip, to detect if a soft-clip is really just an irect

src/core/SkAAClip.cpp

Index: src/core/SkAAClip.cpp
diff --git a/src/core/SkAAClip.cpp b/src/core/SkAAClip.cpp
index 14152f8317ef6a0167b7399d0927528aafd4b990..5d21a47a49470e6f2d85845021cbbed3b76b26bb 100644
--- a/src/core/SkAAClip.cpp
+++ b/src/core/SkAAClip.cpp
@@ -684,6 +684,34 @@ bool SkAAClip::setRect(const SkIRect& bounds) {
 #endif
 }
 
+bool SkAAClip::isRect() const {
+    if (this->isEmpty()) {
+        return false;
+    }
+
+    const RunHead* head = fRunHead;
+    if (head->fRowCount != 1) {
+        return false;
+    }
+    const YOffset* yoff = head->yoffsets();
+    if (yoff->fY != fBounds.fBottom - 1) {
+        return false;
+    }
+
+    const uint8_t* row = head->data() + yoff->fOffset;
+    int width = fBounds.width();
+    do {
+        if (row[1] != 0xFF) {
+            return false;
+        }
+        int n = row[0];
+        SkASSERT(n <= width);
+        width -= n;
+        row += 2;
+    } while (width > 0);
+    return true;
+}
+
 bool SkAAClip::setRect(const SkRect& r, bool doAA) {
     if (r.isEmpty()) {
         return this->setEmpty();