Refactored SkColorSpace and added in a Lab PCS GM

The refactoring breaks off A2B0 tag support into a separate
subclass of SkColorSpace_Base, while keeping the current
(besides CLUT) functionality in a XYZTRC subclass.

ICC profile loading is now aware of this and creates the A2B0
subclass when SkColorSpace::NewICC() is called on a profile
in need of the A2B0 functionality.

The LabPCSDemo GM loads a .icc profile containing a LAB PCS and
then runs a Lab->XYZ conversion on an image using it so we can
display it and test out the A2B0 SkColorSpace functionality,
sans a/b/m-curves, as well as the Lab->XYZ conversion code.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2389983002

Review-Url: https://codereview.chromium.org/2389983002
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 03f03b2..6c99b37 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -6,7 +6,7 @@
  */
 
 #include "Sk4fLinearGradient.h"
-#include "SkColorSpace_Base.h"
+#include "SkColorSpace_XYZ.h"
 #include "SkGradientShaderPriv.h"
 #include "SkHalf.h"
 #include "SkLinearGradient.h"
@@ -1747,7 +1747,8 @@
     if (fUseColors4f) {
         fColorSpace = GrTest::TestColorSpace(random);
         if (fColorSpace) {
-            fColorSpace = as_CSB(fColorSpace)->makeLinearGamma();
+            SkASSERT(SkColorSpace_Base::Type::kXYZ == as_CSB(fColorSpace)->type());
+            fColorSpace = static_cast<SkColorSpace_XYZ*>(fColorSpace.get())->makeLinearGamma();
         }
     }