Introduce GrColorSpaceXform, for gamut conversion on textures

src/gpu/GrColorSpaceXform.cpp

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "GrColorSpaceXform.h"
+#include "SkColorSpace.h"
+
+static inline bool sk_float_almost_equals(float x, float y, float tol) {
+    return sk_float_abs(x - y) <= tol;
+}
+
+static inline bool matrix_is_almost_identity(const SkMatrix44& m,
+                                             SkMScalar tol = SK_MScalar1 / (1 << 12)) {
+    return
+        sk_float_almost_equals(m.getFloat(0, 0), 1.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(0, 1), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(0, 2), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(0, 3), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(1, 0), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(1, 1), 1.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(1, 2), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(1, 3), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(2, 0), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(2, 1), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(2, 2), 1.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(2, 3), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(3, 0), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(3, 1), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(3, 2), 0.0f, tol) &&
+        sk_float_almost_equals(m.getFloat(3, 3), 1.0f, tol);
+}
+
+sk_sp<GrColorSpaceXform> GrColorSpaceXform::Make(SkColorSpace* src, SkColorSpace* dst) {
+    if (!src || !dst) {
+        // Invalid
+        return nullptr;
+    }
+
+    if (src == dst) {
+        // Quick equality check - no conversion needed in this case
+        return nullptr;
+    }
+
+    SkMatrix44 srcToDst(SkMatrix44::kUninitialized_Constructor);
+    if (!dst->xyz().invert(&srcToDst)) {
+        return nullptr;
+    }
+    srcToDst.postConcat(src->xyz());
+
+    if (matrix_is_almost_identity(srcToDst)) {
+        return nullptr;
+    }
+
+    return sk_make_sp<GrColorSpaceXform>(srcToDst);
+}