More blur performance

[p0nce]

The idea is:

• invalidateChars return a rectlist for blur rectangles. To avoid adding too much rectangle, either populate a bitmap of changed chars for blur, and find a span. Or add 1-height rectangles.
• backToPost return another rectlist
• blur happens on two rectlist, dis-overlapped from rectlist enlarge in one or two directions.

Lost my partial implementation :( rather extensive change too.