Add texture compression utility

include/core/SkTypes.h

Index: include/core/SkTypes.h
diff --git a/include/core/SkTypes.h b/include/core/SkTypes.h
index 15de8a4937b37cbd3456a91b52c013c47047068c..eeee33a48ac9d34e0048ad5441b7e218c763a7b4 100644
--- a/include/core/SkTypes.h
+++ b/include/core/SkTypes.h
@@ -354,6 +354,12 @@ template <typename T> inline T SkTAbs(T value) {
     return value;
 }
 
+// Absolute difference between two values. More correct than SkTAbs(a - b)
+// because it works on unsigned values.
+template <typename T> inline T SkTAbsDiff(const T &a, const T &b) {
+    return (a > b)? (a - b) : (b - a);
+}
+
 static inline int32_t SkMax32(int32_t a, int32_t b) {
     if (a < b)
         a = b;