Correct PorterDuff filters.

1. Remove unused modes - makes the class more manageable, and missing
modes can always be readded from the git history.
2. Reuse the existing BlendComposite instances where possible.
3. Fix incorrect alpha computation for multiply mode.
4. Change the alpha computation for all blend modes to compenstate for
the fact that the color filter image that we create extends beyond the
image it is inteded to be applied to.

Change-Id: Iedebf289a23325ee4c6d406dcad46a9edb1855c7