Use glClear to clear the stencil buffer.

Use glClear instead of drawing a rectange so that current layer's mask
for the entire stencil buffer is cleared. This fixes clipping when
drawn to a texture larger than the screen size or when the stencil or
clipping node's children contain vertices with negative coordinates.
Use ccDrawRect to clear stencil buffer on OS X.

Works around a bug in some ATI drivers on 10.7 where glStencilMask
does not affect glClear.

Add a test where the stencil is positioned below the clipping node's origin.
Merge remote-tracking branch 'colesbury/issue1463' into develop-v2
Updates AUTHORS and CHANGELOG