add protected setPreLocked, for pixelrefs who never need the onLockPixels

Mutexes in pixelrefs were done very sloppily initially. The code (a) assumes all
pixelref subclasses want a mutex to guard their lock/unlock virtuals, and (b)
most subclasses use the same mutex for *all* of their instances, even when there
is no explicit need to guard modifying one instances with another.

When we try drawing bitmaps from multiple threads, we are seeing a lot of slow-
down from these mutexes. This CL has two changes to try to speed things up.

1. Add setPreLocked(), for pixelrefs who never need the onLockPixels
virtual to be called. This speeds up those subclasses in multithreaded environs
as it avoids the mutex lock all together (e.g. SkMallocPixelRef).

2. Add setMutex() to allow a subclass to change the mutex choice. ashmem wants
this, since its unflattening constructor cannot pass down the null, it needs
to cleanup afterwards.
Committed: https://code.google.com/p/skia/source/detail?r=3985

[reed1, 2012-05-14 18:05:03]

1. Does this work for you in your multi-threaded case?
2. Is it measurably faster?

[reed1, 2012-05-14 18:05:46]

1.1 Does this work correctly for you in a debug-build with asserts?

[Leon, 2012-05-14 18:47:27]

https://codereview.appspot.com/6199075/diff/2001/src/core/SkMallocPixelRef.cpp
File src/core/SkMallocPixelRef.cpp (right):

https://codereview.appspot.com/6199075/diff/2001/src/core/SkMallocPixelRef.cp...
src/core/SkMallocPixelRef.cpp:61: }
Needs

this->setPreLocked(fStorage, fCTable);

Otherwise we lose the prelocked state when unflattening.

[reed1, 2012-05-14 19:39:36]

change getLockCount() to isLocked(), since the magnitude of the lock-count is
now funny, given the setPreLocked will always return a non-zero lockcount.

[reed1, 2012-05-14 19:51:09]

1. renamed public method for ashmem to useDefaultMutex()
2. s/matrix/mutex

[Leon, 2012-05-15 13:08:31]

https://codereview.appspot.com/6199075/diff/14001/include/core/SkPixelRef.h
File include/core/SkPixelRef.h (right):

https://codereview.appspot.com/6199075/diff/14001/include/core/SkPixelRef.h#n...
include/core/SkPixelRef.h:178: // only call from constructor. Specify a
(possibly) different mutex, or
This comment describes setMutex (below), rather than useDefaultMutex