Separate core and images project.

SkImage calls functions on SkImageDecoder and SkImageEncoder. This
is desired behavior, and it is also desired to include SkImage as
a part of core. In order to keep core from depending on images,
update SkImageDecoder_empty.cpp to implement all of SkImageDecoder
and SkImageEncoder. This file will be built by chrome (in
https://codereview.chromium.org/15960015).

Move force_linking from SkImageDecoder.cpp to its own file. It must
be called to force linking with the image decoders if desired. Call
the function in tools that need it:
sk_image
render_pictures
render_pdfs
sk_hello
filter
bench_pictures
debugger

SkImageDecoder:
Derive from SkNoncopyable, instead of duplicating its
hiding of constructors.

skhello:
Return rather than trying to write a null SkData to the stream.

Revert "Hamfistedly removed core dependence on images"
(commit 0f05f682a90bc125323677abf3476e1027d174f5) and
"Move SkImage::encode to SkImage_Codec.cpp."
(commit 83e47a954d0bf65439f3d9c0c93213063dd70da3.)
These two commits were temporary fixes that this change
cleans up.

SkSnapshot.cpp:
Check for a NULL encoder returned by SkImageEncoder::Create.

BUG=https://code.google.com/p/skia/issues/detail?id=1275
R=djsollen@google.com, robertphillips@google.com

Review URL: https://codereview.chromium.org/15806010

git-svn-id: http://skia.googlecode.com/svn/trunk@9364 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/images/SkForceLinking.h b/include/images/SkForceLinking.h
new file mode 100644
index 0000000..39901f6
--- /dev/null
+++ b/include/images/SkForceLinking.h
@@ -0,0 +1,20 @@
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/**
+ *  This function's sole purpose is to trick the linker into not discarding
+ *  SkImageDecoder subclasses just because we do not directly call them.
+ *  This is necessary in applications that will create image decoders from
+ *  a stream.
+ *  Call this function with an expression that evaluates to false to ensure
+ *  that the linker includes the subclasses.
+ *  Passing true will result in leaked objects.
+ */
+int SkForceLinking(bool doNotPassTrue);
+
+#define __SK_FORCE_IMAGE_DECODER_LINKING       \
+static int linking_forced = SkForceLinking(false)