add command line flags

add command line flags

BUG=
R=djsollen@google.com

Committed: https://code.google.com/p/skia/source/detail?r=9794

[djsollen, 2013-06-26 14:34:02]

https://codereview.chromium.org/17885003/diff/1/experimental/skpdiff/main.cpp
File experimental/skpdiff/main.cpp (right):

https://codereview.chromium.org/17885003/diff/1/experimental/skpdiff/main.cpp...
experimental/skpdiff/main.cpp:19: 
remove

https://codereview.chromium.org/17885003/diff/1/experimental/skpdiff/main.cpp...
experimental/skpdiff/main.cpp:29: DEFINE_int32(differ, 0, "The index of the
differ to use. Uses 0 if not given.");
instead of using an index it will be much clearer if you give a string name.

out/Debug/pdiff -d histogram -f ...

Also should I be able to select more than one diff metric per invocation? 
Should the default be all metrics?

[djsollen, 2013-06-27 12:43:50]

https://codereview.chromium.org/17885003/diff/11001/experimental/skpdiff/README
File experimental/skpdiff/README (right):

https://codereview.chromium.org/17885003/diff/11001/experimental/skpdiff/READ...
experimental/skpdiff/README:10: out/Debug/skpdiff --differ 1 -f \
update this

https://codereview.chromium.org/17885003/diff/11001/experimental/skpdiff/main...
File experimental/skpdiff/main.cpp (right):

https://codereview.chromium.org/17885003/diff/11001/experimental/skpdiff/main...
experimental/skpdiff/main.cpp:158: SkDifferentPixelsImageDiffer gDiffPixel;
why does this or the 2 arrays below need to be global?

Ideally we would have these differ's self register (like GMs and Tests do), but
lets not spend time on the registration in this CL.  Just add a TODO about that
and we can come back to it later if we have time.

https://codereview.chromium.org/17885003/diff/11001/experimental/skpdiff/main...
experimental/skpdiff/main.cpp:176: SkDebugf("Differs:\n");
"available metrics" as the user facing way to describe the internal concept of a
differ

https://codereview.chromium.org/17885003/diff/11001/experimental/skpdiff/main...
experimental/skpdiff/main.cpp:222: for (int differIndex = 0; differIndex <
chosenDiffers.count(); differIndex++) {
This is going to be a pretty inefficient method of doing the differences as
you're going to decode the baselines for each diff.  Let's not attempt to solve
that this CL either, but again add a TODO describing the inefficiency and follow
up with another CL to fix it.

https://codereview.chromium.org/17885003/diff/11001/experimental/skpdiff/main...
experimental/skpdiff/main.cpp:227: // Initialize the differ using the global
list of init functions that match the list of differs
exceeds 100 char

[djsollen, 2013-06-27 13:35:36]

lgtm

[Zach Reizner, 2013-06-27 17:51:39]

Committed patchset #4 manually as r9794 (presubmit successful).