change typeface::purgeall to not assume that the array is empty, as it may

change typeface::purgeall to not assume that the array is empty, as it may
contain typefaces that still are referenced.

change SkTypeface::UniqueID to internally hold onto a reference to the default
typeface, so that it will always be available if the font cache tries to resolve
it. Then general rule being: if we have a fontID specified by SkPaint, then its
associated typeface object must be alive.

git-svn-id: http://skia.googlecode.com/svn/trunk@2889 2bbb7eff-a529-9590-31e7-b0007b416f81