change write-image to use pipeverbs like everyone else, and add test

src/pipe/SkPipeFormat.h

Index: src/pipe/SkPipeFormat.h
diff --git a/src/pipe/SkPipeFormat.h b/src/pipe/SkPipeFormat.h
index f521657a7c89d3f04cb7f825fe07567c2b91ffcc..539d385fab765a762823e8593186692bb3b9a0cb 100644
--- a/src/pipe/SkPipeFormat.h
+++ b/src/pipe/SkPipeFormat.h
@@ -11,7 +11,6 @@
 #include "SkTypes.h"
 
 #define kDefinePicture_ExtPipeVerb  SkSetFourByteTag('s', 'k', 'p', 'i')
-#define kDefineImage_ExtPipeVerb    SkSetFourByteTag('s', 'k', 'i', 'm')
 
 enum class SkPipeVerb : uint8_t {
     kSave,              // extra == 0
@@ -57,6 +56,7 @@ enum class SkPipeVerb : uint8_t {
     kDefineFactory,     // extra == factory_index (followed by padded getTypeName string)
     kDefinePicture,     // extra == 0 or forget_index + 1 (0 means we're defining a new picture)
     kEndPicture,        // extra == picture_index
+    kWriteImage,        // extra == image_index
 };
 
 enum PaintUsage {