viewport.rs: in tests, use numbers that can be represented exactly as f64
viewport.rs: in tests, use numbers that can be represented exactly as f64

In clip_to_viewport(), we were expecting a result of 0.2, which can't
be represented exactly.  So, the resulting matrix with 0.2 components
in it wasn't being compared successfully on i386.

(why was it fine on x86_64?)

We'll need some bigger guns for asserting on float "equality", it seems.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg