st-scroll-view-fade: Don't fade out gradient

st-scroll-view-fade: Don't fade out gradient

The changes from commit b4f5f1e4617434 and b394d184cc7fa6 increased the
instructions required for the fade fragment shader. This is over the limit
for some hardware (like intel gen3), which causes the driver to fallback
to software rendering for the shader. The result is that painting a scrollview
that has a fade effect takes around 30 (!!) seconds.

So lets go back to the old effect for 3.8 until we find a solution.

https://bugzilla.gnome.org/show_bug.cgi?id=696404