Fix persistent Canvas clip handling

Partial Canvas save semantics (clip or matrix persisting after restore)
are currently emulated in the native canvas wrapper (SkiaCanvas.cpp).

Persistent clips (save w/ MATRIX_SAVE_FLAG only) in particular are
emulated using the SkCanvas clip stack.  There are two problems with
the current implementation:

1) The canvas save count is used to identify the clip stack topmost
frame, on the assumption that it is the same as the actual clip stack
save count.  But with the introduction of lazy SkCanvas saves in Skia,
the two values can diverge: the clip stack save count only reflects
*committed* saves, while the canvas save count includes both committed
and pending saves.  This means that we can no longer compare canvas and
clip stack save counts directly.

While we're looking at addressing the save count discrepancy in Skia
proper, we can also refactor the partial save emulation to no longer
rely on the two values being synchronized: instead of using the canvas
save count to locate the top clip stack frame, simply use the clip
stack save count for the same purpose - getClipStack()->getSaveCount()
always points to the correct top clip stack frame.

With this patch:

  * we use SkCanvas::getSaveCount() to track *canvas* save frames which
      require persistent matrix/clip handling (mSaveRecStack)
  * we use SkClipStack::getSaveCount() to extract the clips from the
    top clip stack frame

Also, since we're no longer mixing/comparing the two save counts, we
don't have to decrement the canvas value anymore (to make it zero-based
like its clip stack counterpart).

2) When iterating over clip stack elements, we currently start at
kTopIterStart and advance using next().  This is incorrect as next()
moves up the stack, so we only iterate over the topmost element =>
if there are multiple (non-consolidated) clip elements in the top
frame, we only get to see one.

We need to iterate using prev() instead.

Change-Id: Ic2d8cad684018925e749b9172fbf7c6202d9fb62