add SkMatrix::decomposeScale

tests/MatrixTest.cpp

Index: tests/MatrixTest.cpp
diff --git a/tests/MatrixTest.cpp b/tests/MatrixTest.cpp
index 7875d9ea45cd5e726cde798521eff1c6f1d25739..95d33acabd7ac42e0545efbc93b2a84d32ca668b 100644
--- a/tests/MatrixTest.cpp
+++ b/tests/MatrixTest.cpp
@@ -771,6 +771,34 @@ static void test_matrix_homogeneous(skiatest::Reporter* reporter) {
 
 }
 
+static bool check_decompScale(const SkMatrix& matrix) {
+    SkSize scale;
+    SkMatrix remaining;
+
+    if (!matrix.decomposeScale(&scale, &remaining)) {
+        return false;
+    }
+    if (scale.width() <= 0 || scale.height() <= 0) {
+        return false;
+    }
+    remaining.preScale(scale.width(), scale.height());
+    return nearly_equal(matrix, remaining);
+}
+
+static void test_decompScale(skiatest::Reporter* reporter) {
+    SkMatrix m;
+
+    m.reset();
+    REPORTER_ASSERT(reporter, check_decompScale(m));
+    m.setScale(2, 3);
+    REPORTER_ASSERT(reporter, check_decompScale(m));
+    m.setRotate(35, 0, 0);
+    REPORTER_ASSERT(reporter, check_decompScale(m));
+
+    m.setScale(1, 0);
+    REPORTER_ASSERT(reporter, !check_decompScale(m));
+}
+
 DEF_TEST(Matrix, reporter) {
     SkMatrix    mat, inverse, iden1, iden2;
 
@@ -889,6 +917,8 @@ DEF_TEST(Matrix, reporter) {
     test_matrix_decomposition(reporter);
     test_matrix_homogeneous(reporter);
     test_set9(reporter);
+
+    test_decompScale(reporter);
 }
 
 DEF_TEST(Matrix_Concat, r) {