SkMatrix44 clarifications and clean-ups

Fixed row/col major bug and added comments to the
header.

Made SkMatrix::I() thread safe using constexpr.

Added a test set3x3RowMajorf().

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2098583002

Review-Url: https://codereview.chromium.org/2098583002
diff --git a/src/codec/SkPngCodec.cpp b/src/codec/SkPngCodec.cpp
index 985d61a..32308b2 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -230,7 +230,7 @@
             toXYZD50[i] = png_fixed_point_to_float(XYZ[i]);
         }
         SkMatrix44 mat(SkMatrix44::kUninitialized_Constructor);
-        mat.set3x3ColMajorf(toXYZD50);
+        mat.set3x3RowMajorf(toXYZD50);
 
         if (PNG_INFO_gAMA == png_get_gAMA_fixed(png_ptr, info_ptr, &gamma)) {
             float value = png_inverted_fixed_point_to_float(gamma);
@@ -257,7 +257,7 @@
 
         // Since there is no cHRM, we will guess sRGB gamut.
         SkMatrix44 mat(SkMatrix44::kUninitialized_Constructor);
-        mat.set3x3ColMajorf(gSRGB_toXYZD50);
+        mat.set3x3RowMajorf(gSRGB_toXYZD50);
 
         return SkColorSpace_Base::NewRGB(gammas, mat);
     }
diff --git a/src/core/SkColorSpace.cpp b/src/core/SkColorSpace.cpp
index 95c38f2..3f93f16 100644
--- a/src/core/SkColorSpace.cpp
+++ b/src/core/SkColorSpace.cpp
@@ -142,7 +142,7 @@
         case kSRGB_Named: {
             sRGBOnce([] {
                 SkMatrix44 srgbToxyzD50(SkMatrix44::kUninitialized_Constructor);
-                srgbToxyzD50.set3x3ColMajorf(gSRGB_toXYZD50);
+                srgbToxyzD50.set3x3RowMajorf(gSRGB_toXYZD50);
                 sRGB = new SkColorSpace_Base(kSRGB_GammaNamed, srgbToxyzD50, kSRGB_Named);
             });
             return sk_ref_sp(sRGB);
@@ -150,7 +150,7 @@
         case kAdobeRGB_Named: {
             adobeRGBOnce([] {
                 SkMatrix44 adobergbToxyzD50(SkMatrix44::kUninitialized_Constructor);
-                adobergbToxyzD50.set3x3ColMajorf(gAdobeRGB_toXYZD50);
+                adobergbToxyzD50.set3x3RowMajorf(gAdobeRGB_toXYZD50);
                 adobeRGB = new SkColorSpace_Base(k2Dot2Curve_GammaNamed, adobergbToxyzD50,
                                                  kAdobeRGB_Named);
             });
@@ -897,7 +897,7 @@
                     return_null("Need valid rgb tags for XYZ space");
                 }
                 SkMatrix44 mat(SkMatrix44::kUninitialized_Constructor);
-                mat.set3x3ColMajorf(toXYZ);
+                mat.set3x3RowMajorf(toXYZ);
 
                 // It is not uncommon to see missing or empty gamma tags.  This indicates
                 // that we should use unit gamma.
diff --git a/src/core/SkMatrix44.cpp b/src/core/SkMatrix44.cpp
index 56c2e8a..83c1adc 100644
--- a/src/core/SkMatrix44.cpp
+++ b/src/core/SkMatrix44.cpp
@@ -186,7 +186,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 const SkMatrix44& SkMatrix44::I() {
-    static const SkMatrix44 gIdentity44(kIdentity_Constructor);
+    static constexpr SkMatrix44 gIdentity44(kIdentity_Constructor);
     return gIdentity44;
 }
 
@@ -220,7 +220,7 @@
     this->dirtyTypeMask();
 }
 
-void SkMatrix44::set3x3ColMajorf(const float src[]) {
+void SkMatrix44::set3x3RowMajorf(const float src[]) {
     fMat[0][0] = src[0]; fMat[0][1] = src[3]; fMat[0][2] = src[6]; fMat[0][3] = 0;
     fMat[1][0] = src[1]; fMat[1][1] = src[4]; fMat[1][2] = src[7]; fMat[1][3] = 0;
     fMat[2][0] = src[2]; fMat[2][1] = src[5]; fMat[2][2] = src[8]; fMat[2][3] = 0;