SkImageEncoder: simplify API

(re-land 248ff02 & 2cb6cb7, with changes)

  - Hide SkImageEncoder class in private header.
  - SkImageEncoder::Type becomes SkEncodedImageFormat
  - SkEncodedFormat becomes SkEncodedImageFormat
  - SkImageEncoder static functions replaced with
    single function EncodeImage()
  - utility wrappers for EncodeImage() are in
    sk_tool_utils.h

TODO: remove link-time registration mechanism.
TODO: clean up clients use of API and flip the flag.
TODO: implement EncodeImage() in chromeium/skia/ext

Change-Id: I47d451e50be4d5c6c130869c7fa7c2857243d9f0
Reviewed-on: https://skia-review.googlesource.com/4909
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-on: https://skia-review.googlesource.com/5186
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>
diff --git a/gm/image_shader.cpp b/gm/image_shader.cpp
index 924199e..49b1ed5 100644
--- a/gm/image_shader.cpp
+++ b/gm/image_shader.cpp
@@ -55,7 +55,7 @@
     if (!src) {
         return nullptr;
     }
-    sk_sp<SkData> encoded(src->encode(SkImageEncoder::kPNG_Type, 100));
+    sk_sp<SkData> encoded(src->encode(SkEncodedImageFormat::kPNG, 100));
     if (!encoded) {
         return nullptr;
     }