add friendly new ctor for xform steps

This isn't even used, so it definitely definitely
will have no perf impact.

Change-Id: I3e459bce440aef5e3e8fa2724dd2839b866920b6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257656
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

src/core/SkColorSpaceXformSteps.h

@@ -38,6 +38,11 @@ struct SkColorSpaceXformSteps {
     SkColorSpaceXformSteps(SkColorSpace* src, SkAlphaType srcAT,
                            SkColorSpace* dst, SkAlphaType dstAT);
 
+    template <typename S, typename D>
+    SkColorSpaceXformSteps(const S& src, const D& dst)
+        : SkColorSpaceXformSteps(src.colorSpace(), src.alphaType(),
+                                 dst.colorSpace(), dst.alphaType()) {}
+
     void apply(float rgba[4]) const;
     void apply(SkRasterPipeline*, bool src_is_normalized) const;