Add preliminary ASTC encoder

src/utils/SkTextureCompressor.h

Index: src/utils/SkTextureCompressor.h
diff --git a/src/utils/SkTextureCompressor.h b/src/utils/SkTextureCompressor.h
index a7afe0384b041f196b3bccb4baf1d66e32241d34..5238ba221be035c78d118b78346dee5fc788f0d6 100644
--- a/src/utils/SkTextureCompressor.h
+++ b/src/utils/SkTextureCompressor.h
@@ -18,10 +18,11 @@ namespace SkTextureCompressor {
     // Various texture compression formats that we support.
     enum Format {
         // Alpha only formats.
-        kLATC_Format,    // 4x4 blocks, compresses A8
-        kR11_EAC_Format, // 4x4 blocks, compresses A8
+        kLATC_Format,       // 4x4 blocks, compresses A8
+        kR11_EAC_Format,    // 4x4 blocks, compresses A8
+        kASTC_12x12_Format, // 12x12 blocks, compresses A8
 
-        kLast_Format = kR11_EAC_Format
+        kLast_Format = kASTC_12x12_Format
     };
     static const int kFormatCnt = kLast_Format + 1;