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/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;