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:

Review URL: https://codereview.chromium.org/759653004
Sk4x_sse.h

CQ_EXTRA_TRYBOTS=client.skia:Test-Mac10.6-MacMini4.1-GeForce320M-x86_64-Release-Trybot,Test-Mac10.7-MacMini4.1-GeForce320M-x86_64-Release-Trybot,Test-Win7-ShuttleA-HD2000-x86-Debug-GDI-Trybot

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/e4bf793120d3bfc9b003d11880a3fb73ff2b89e9

Review URL: https://codereview.chromium.org/698873003
Add IndexType parameter to GrDrawTarget::drawPaths

Allows the caller to decide whether they sent 8, 16, or 32 bit path
indices.

BUG=skia:

Review URL: https://codereview.chromium.org/746253003