SkColorSpaceXform bug fixes attempt 2

SkColorSpaceXform bug fixes attempt 2

(1) Clamping

If we're going to clamp (8888 outputs), we need to clamp properly
to alpha (not 1) when we premultiply.  This fix is made in
SkColorSpaceXform_XYZ.

An alternative fix would move all clamping out of the store
functions, to before the gamma encoding.  This generally makes sense,
but the "to 2.2 conversion" may introduce NaNs and always needs a
clamp.  So another fix is to just have an extra clamp in the store 2.2
function.  Since we have two pipelines, let's try this one in
SkColorSpaceXform_Pipeline :).

(2) Correctly handle the memcpy() case.

This is not changed from a previous (reverted) CL.

Looks like this only ever worked for RGBA inputs,
never got updated when we added BGRA inputs.

This probably flew under the radar because the
clients are smart enough to avoid performing a
color xform altogether when the color spaces
match.

BUG=skia:

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: I0b59239d2488ce9fdbe11efbd96567e420bb9813
Reviewed-on: https://skia-review.googlesource.com/5464
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>