StScrollViewFade: Fix GLSL shader to work on r300 hardware

The number instructions in a shader is limited to 64 on r300 hardware,
the fade shader in StScrollViewFade was ending up using 97 instructions
which is way over the limit.

So refactor the shader to use less instructions by precomputing as many
values as possible outside of the conditionals. The resulting shader
ends up using 34 instructions which is well within the hardware limits.

https://bugzilla.gnome.org/show_bug.cgi?id=644589