Improve PorterDuff support.

1. Don't cache the src image used for applying the filter. The filter
delegate is not always cleared. This probably results in slighly slower
rendering, but doesn't run Studio out of memory.
2. Support more PorterDuff modes.
3. Fix a bug where the alpha was applied twice and thus the filter had
less effect than it should have had.

Change-Id: I2a481a64ba7f1ff8e9683bbc46ae110433e82ebc