Modify SkBitmap::extractSubset() to respect opaqueness

The resulting subset bitmap will always have the same opaqueness flag as the
source bitmap.

BUG=439
Review URL: http://codereview.appspot.com/5534051

git-svn-id: http://skia.googlecode.com/svn/trunk@3036 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed