add SkMatrixPriv for specialized helpers

src/core/SkMatrix.cpp

Index: src/core/SkMatrix.cpp
diff --git a/src/core/SkMatrix.cpp b/src/core/SkMatrix.cpp
index fb0c69d7c7287d8428e9de233c85e38280374a69..a05ae0a177670a298b4945e870a534d7c584183f 100644
--- a/src/core/SkMatrix.cpp
+++ b/src/core/SkMatrix.cpp
@@ -1102,6 +1102,15 @@ void SkMatrix::mapVectors(SkPoint dst[], const SkPoint src[], int count) const {
     }
 }
 
+static Sk4f sort_as_rect(const Sk4f& ltrb) {
+    Sk4f rblt(ltrb[2], ltrb[3], ltrb[0], ltrb[1]);
+    Sk4f min = Sk4f::Min(ltrb, rblt);
+    Sk4f max = Sk4f::Max(ltrb, rblt);
+    // We can extract either pair [0,1] or [2,3] from min and max and be correct, but on
+    // ARM this sequence generates the fastest (a single instruction).
+    return Sk4f(min[2], min[3], max[0], max[1]);
+}
+
 void SkMatrix::mapRectScaleTranslate(SkRect* dst, const SkRect& src) const {
     SkASSERT(dst);
     SkASSERT(this->isScaleTranslate());
@@ -1112,20 +1121,19 @@ void SkMatrix::mapRectScaleTranslate(SkRect* dst, const SkRect& src) const {
     SkScalar ty = fMat[kMTransY];
     Sk4f scale(sx, sy, sx, sy);
     Sk4f trans(tx, ty, tx, ty);
-
-    Sk4f ltrb = Sk4f::Load(&src.fLeft) * scale + trans;
-    // need to sort so we're not inverted
-    Sk4f rblt(ltrb[2], ltrb[3], ltrb[0], ltrb[1]);
-    Sk4f min = Sk4f::Min(ltrb, rblt);
-    Sk4f max = Sk4f::Max(ltrb, rblt);
-    // We can extract either pair [0,1] or [2,3] from min and max and be correct, but on
-    // ARM this sequence generates the fastest (a single instruction).
-    Sk4f(min[2], min[3], max[0], max[1]).store(&dst->fLeft);
+    sort_as_rect(Sk4f::Load(&src.fLeft) * scale + trans).store(&dst->fLeft);
 }
 
 bool SkMatrix::mapRect(SkRect* dst, const SkRect& src) const {
     SkASSERT(dst);
 
+    if (this->getType() <= kTranslate_Mask) {
+        SkScalar tx = fMat[kMTransX];
+        SkScalar ty = fMat[kMTransY];
+        Sk4f trans(tx, ty, tx, ty);
+        sort_as_rect(Sk4f::Load(&src.fLeft) + trans).store(&dst->fLeft);
+        return true;
+    }
     if (this->isScaleTranslate()) {
         this->mapRectScaleTranslate(dst, src);
         return true;