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/gyp/core.gypi b/gyp/core.gypi
index ab73ede..534c888 100644
--- a/gyp/core.gypi
+++ b/gyp/core.gypi
@@ -189,7 +189,7 @@
         '<(skia_src_path)/image/SkDataPixelRef.cpp',
         '<(skia_src_path)/image/SkImage.cpp',
         '<(skia_src_path)/image/SkImagePriv.cpp',
-#        '<(skia_src_path)/image/SkImage_Codec.cpp',
+        '<(skia_src_path)/image/SkImage_Codec.cpp',
 #        '<(skia_src_path)/image/SkImage_Gpu.cpp',
         '<(skia_src_path)/image/SkImage_Picture.cpp',
         '<(skia_src_path)/image/SkImage_Raster.cpp',
@@ -232,6 +232,8 @@
         '<(skia_include_path)/core/SkFontHost.h',
         '<(skia_include_path)/core/SkGeometry.h',
         '<(skia_include_path)/core/SkGraphics.h',
+        '<(skia_include_path)/core/SkImageDecoder.h',
+        '<(skia_include_path)/core/SkImageEncoder.h',
         '<(skia_include_path)/core/SkImageFilter.h',
         '<(skia_include_path)/core/SkImageFilterUtils.h',
         '<(skia_include_path)/core/SkInstCnt.h',