Sk_API for SkImageGenerator and SkInstallDiscardablePixelRef

Added SK_API to SkImageGenerator (already in include/).

Moved SkDiscardablePixelRef::Install to SkInstallDiscardablePixelRef,
added SK_API to that function, and moved declaration to
SkImageGenerator.h

This keeps the SkDiscardablePixelRef internal to Skia, but exposes a
method to install it into a bitmap.

Modifed tests that rely on this functio to use new version.

BUG=
R=mtklein@google.com, reed@google.com

Review URL: https://codereview.chromium.org/111713002

git-svn-id: http://skia.googlecode.com/svn/trunk@12612 2bbb7eff-a529-9590-31e7-b0007b416f81
7 files changed