Remove float comparisons with ==

We do some float comparisons with ==, which is wrong in some cases, and the clippy lints rightfully complain:

warning: strict comparison of f32 or f64
   --> rsvg_internals/src/path_builder.rs:127:24
    |
127 |         if x1 == x2 && y1 == y2 {

warning: strict comparison of f32 or f64
   --> rsvg_internals/src/pattern.rs:417:27
    |
417 |     if scwscale != 1.0 || schscale != 1.0 {

In the first case (arbitrary x1 == x2), see this issue against float-cmp, about comparing floats against (possibly) zero. We can experiment with having a local ApproxEqEpsilonOrUlps trait, with the epsilon adjusted to the smallest fixed-point number that Cairo can represent. Or something like that: each comparison needs to be considered independently.

In the second case (scwscale != 1.0), I think we can just use float_cmp::ApproxEqUlps since we are not comparing against zero.