Don't rely on implicit type conversion in the scroll view shader

Don't rely on implicit type conversion in the scroll view shader

Before version 1.2 of GLSL it would not implicitly convert from int to
float which meant that if you compare a float variable with an integer
constant it will generate a compile error. In particular this means
that on GLES2 (which uses GLSL 1.0) the scroll view shader will not
compile on pedantic compilers, which includes Mesa. This patch just
changes it to use floating point constants.

https://bugzilla.gnome.org/show_bug.cgi?id=693339