First attempt at fixing the 'straight' blend method. It now applies to the whole...

First attempt at fixing the 'straight' blend method.  It now applies to the whole layer, rather than just the pixels modified by the layer.  Loading old canvases that use 'straight' will convert the blend method to 'composite' for all layers other than PasteCanvas, since the old 'straight' worked much like composite for them.  The 'straight onto' blend method still needs work.

git-svn-id: http://svn.voria.com/code@1422 1f10aa63-cdf2-0310-b900-c93c546f37ac