st-scroll-view-fade: Reduce number of GLSL uniforms and instructions

The vvalue and hvalue uniforms are only used to decide whether we
should do fade the edges or not based on the fade_edges uniform.
The result does not change accross fragments so there is no reason
to recompute it for every fragment (pixel) so just split the edge
fade into two uniforms and compute the "should we fade the edges"
boolean once for every direction (when setting the uniforms) instead
of for every single fragment twice.

This reduces the number of uniforms as well as the the number of instructions
which are limited on older hardware. It should also be more efficent.

https://bugzilla.gnome.org/show_bug.cgi?id=708007