Fixes #881

Unfortunately only implemented for shadings where the 3 vertices of the
triangle have the same color for now since i got lost trying to
implement the coloring (and also have no pdf to check against)

The reason this fixes #881 is because if Splash::gouraudTriangleShadedFill
returns false because it doesn't natively support this shading, the
default rendering algorithm of Gfx.cc kicks in, and that rendering what
does is render different triangles without them knowing they belong to
the same shading, meaning that if you have some opacity the edges of the
triangles will overlap and and up having different color than the one
you really wanted