Update invariant output computation for various texture effects.

Update invariant output computation for various texture effects.

Update various effects that read in textures to check whether or not the texture is alpha only.
This allows us to use a more specific mulByUnKnownAlpha instead of the more general mulByUnknownColor

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/f8449babdc430bb522c0eb87ce18f05c54d83714

[egdaniel, 2014-11-25 17:04:41]

egdaniel@google.com changed reviewers:
+ jvanverth@google.com

[egdaniel, 2014-11-25 17:04:41]

This improvement is needed so that when I land my XP work I can correctly
distinguish if an LCD effect is installed. The LCD effect should be the only
effects that force us to 4 channel coverage output. However mulByUnknownColor()
forces us to 4 channel where as mulByUnknownAlpha will keep whatever the current
number of channels is. With this change the invariant output calculations will
only give us four channel with LCD.

[jvanverth1, 2014-11-25 18:13:43]

lgtm

[egdaniel, 2014-11-25 18:15:07]

The CQ bit was checked by egdaniel@google.com

[commit-bot: I haz the power, 2014-11-25 18:15:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/759653004/1

[commit-bot: I haz the power, 2014-11-25 18:25:00]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/f8449babdc430bb522c0eb87ce18f05c54d83714