Fix filter optimizations to take the paint's alpha into account

This doesn't change anything in the skp library but, since the Apply functions don't handle the two alpha case, I thought it best to restrict the optimizations.

[reed1, 2013-02-12 15:05:03]

lgtm, but w/ a suggestion to rewrite a little for more clarity/simplicity. Also,
it would offer an easy place to add a comment to document *why* we want to
consider the layer's paint color to be opaque...

https://codereview.appspot.com/7312083/diff/1/tools/filtermain.cpp
File tools/filtermain.cpp (right):

https://codereview.appspot.com/7312083/diff/1/tools/filtermain.cpp#newcode60
tools/filtermain.cpp:60: 
I find this 4-lines-of-compoent-gets very hard to read for correctness (though I
think it is correct). Seems like we could have restructured this test a little
for more clarity.

e.g.

SkColor layerColor = saveLayerPaint->getColor() | 0xFF000000; // force opaque

if (... && dbmrPaint->getColor() == layerColor && ...)

[robertphillips, 2013-02-12 15:27:17]

Added comments

https://codereview.appspot.com/7312083/diff/1/tools/filtermain.cpp
File tools/filtermain.cpp (right):

https://codereview.appspot.com/7312083/diff/1/tools/filtermain.cpp#newcode60
tools/filtermain.cpp:60: 
Thanks - that is a lot clearer.

[robertphillips, 2013-02-13 13:27:58]

committed as r7711