Fix bug in cubic-clipper (SkEdgeClipper). When we chop the cubic on Top/Bottom

Fix bug in cubic-clipper (SkEdgeClipper). When we chop the cubic on Top/Bottom
of the cliprect, we (correctly) clamp the Y coordinate of the control-point
right next to the on-curve point that was chopped (this ensures we don't go
slightly outside of the clip-rect due to imperfect T value calculation). However,
the code was also clamping the other control-point as well, resulting in warping
the cubic, which could sometimes force it outside of the clip.

The fix is to just remove the line of code that clampped the 2nd control-point.

unittest added to reproduce a test cubic that triggered an assert, due to the
cubic being outside of the cliprect. The test (w/o the fix) will assert in 
a SK_DEBUG build.
Committed: https://code.google.com/p/skia/source/detail?r=7184

[caryclark1, 2013-01-15 15:48:00]

lgtm