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/gyp@9364 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/images.gyp b/images.gyp
index d63a594..44f944c 100644
--- a/images.gyp
+++ b/images.gyp
@@ -25,8 +25,7 @@
         '../src/image/',
       ],
       'sources': [
-        '../include/images/SkImageDecoder.h',
-        '../include/images/SkImageEncoder.h',
+        '../include/images/SkForceLinking.h',
         '../include/images/SkImageRef.h',
         '../include/images/SkImageRef_GlobalPool.h',
         '../src/images/SkJpegUtility.h',
@@ -38,11 +37,12 @@
 
         '../src/images/SkBitmapRegionDecoder.cpp',
 
+        '../src/images/SkForceLinking.cpp',
         '../src/images/SkImageDecoder.cpp',
         '../src/images/SkImageDecoder_FactoryDefault.cpp',
         '../src/images/SkImageDecoder_FactoryRegistrar.cpp',
         # If decoders are added/removed to/from (all/individual)
-        # platform(s), be sure to update SkImageDecoder.cpp:force_linking
+        # platform(s), be sure to update SkForceLinking.cpp
         # so the right decoders will be forced to link.
         '../src/images/SkImageDecoder_libbmp.cpp',
         '../src/images/SkImageDecoder_libgif.cpp',