core plugin: suppress 'offset' double comparison warning

core plugin: suppress 'offset' double comparison warning

This code is trying to abbreviate printing `0.000` or `1.000`. We can more
accurately describe the situations under which this will happen, squashing:

  gvrender_core_svg.c: In function ‘svg_print_stop’:
  gvrender_core_svg.c:525:16: warning: comparing floating-point with ‘==’ or
    ‘!=’ is unsafe [-Wfloat-equal]
    525 |     if (offset == 0.0)
        |                ^~
  gvrender_core_svg.c:527:21: warning: comparing floating-point with ‘==’ or
    ‘!=’ is unsafe [-Wfloat-equal]
    527 |     else if (offset == 1.0)
        |                     ^~