add SkMatrix::decomposeScale

src/core/SkMatrix.cpp

Index: src/core/SkMatrix.cpp
diff --git a/src/core/SkMatrix.cpp b/src/core/SkMatrix.cpp
index 9658177ec913e3b3e091104d31224e798532df70..0c7f110b13dc16cd9e549e59af7261c1b6a46df5 100644
--- a/src/core/SkMatrix.cpp
+++ b/src/core/SkMatrix.cpp
@@ -1603,6 +1603,27 @@ const SkMatrix& SkMatrix::InvalidMatrix() {
     return invalid.asSkMatrix();
 }
 
+bool SkMatrix::decomposeScale(SkSize* scale, SkMatrix* remaining) const {
+    if (this->hasPerspective()) {
+        return false;
+    }
+
+    const SkScalar sx = SkVector::Length(this->getScaleX(), this->getSkewY());
+    const SkScalar sy = SkVector::Length(this->getSkewX(), this->getScaleY());
+    if (0 == sx || 0 == sy) {
+        return false;
+    }
+
+    if (scale) {
+        scale->set(sx, sy);
+    }
+    if (remaining) {
+        *remaining = *this;
+        remaining->postScale(SkScalarInvert(sx), SkScalarInvert(sy));

[Stephen White, 2015/03/19 18:27:50]

Hmm.. for my uses, I was also extracting the translate component from the
remainder matrix, so that the CTM looks at least positionally similar to what it
was before. Now the translation will be applied by the SkMatrixImageFilter, and
the CTM will always have 0, 0 translation. I guess that's ok, it'll just look a
little weird during debugging.

+    }
+    return true;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 size_t SkMatrix::writeToMemory(void* buffer) const {