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/tools/render_pdfs_main.cpp b/tools/render_pdfs_main.cpp
index f2ce0a5..1821548 100644
--- a/tools/render_pdfs_main.cpp
+++ b/tools/render_pdfs_main.cpp
@@ -7,6 +7,7 @@
 
 #include "SkCanvas.h"
 #include "SkDevice.h"
+#include "SkForceLinking.h"
 #include "SkGraphics.h"
 #include "SkImageDecoder.h"
 #include "SkImageEncoder.h"
@@ -17,6 +18,8 @@
 #include "PdfRenderer.h"
 #include "picture_utils.h"
 
+__SK_FORCE_IMAGE_DECODER_LINKING;
+
 #ifdef SK_USE_CDB
 #include "win_dbghelp.h"
 #endif