Add downsample from 8888 to 4444.

Extend SkBitmap::copyTo to copy from a source with
SkARGB_8888_Config to a destination bitmap with
SkARGB_4444_Config.

BUG=http://code.google.com/p/chromium/issues/detail?id=245774
R=reed@google.com

Review URL: https://codereview.chromium.org/22350003

git-svn-id: http://skia.googlecode.com/svn/trunk@10621 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed