Fix SkMatrix44::invert(...) for identity matrices and a NULL parameter

Fix SkMatrix44::invert(...) for identity matrices and a NULL parameter

Currently, to check if an SkMatrix44 is singular, we call ::invert(NULL) and
check the result. Usually this does no more work than is necessary. One
case where we do far too much work is if |this| is an identity matrix. In
this case, we do not early out and compute a (partial) determinant (we don't
do the full shebang because there are no perspective components).

BUG=None

Committed: http://code.google.com/p/skia/source/detail?r=12327

[Ian Vollick, 2013-11-20 20:03:05]

+ajuma for correctness.
+reed for OWNERS.

[ajuma, 2013-11-20 20:06:19]

lgtm

[reed1, 2013-11-20 20:19:46]

lgtm

[commit-bot: I haz the power, 2013-11-20 20:28:31]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/vollick@chromium.org/79333002/1

[commit-bot: I haz the power, 2013-11-20 21:32:30]

Change committed as 12327