Plug-ins: a filter's gradient stop plates are clamped into range like its other colors

A filter's or importer's output clamps each gradient stop's offset,
opacity, midpoint, tint and color, but kept its four process plates as
given, so a plate below 0, above 1 or infinite was stored in the
document. Each plate is now clamped to 0-1: a value that is not a
number becomes 0, and an infinite one becomes 0 or 1 by its sign.
Painted live effects still refuse such a stop.

Docs: docs/plugins.md, docs/target-app-parity.md.

crates/plugins/src/objects.rs

@@ -579,6 +579,11 @@ fn unit(v: f32) -> f32 {
     v.clamp(0.0, 1.0)
 }
 
+/// Process plate `v` in 0..1: NaN is 0, infinities go to the nearest end.
+fn plate(v: f32) -> f32 {
+    if v.is_nan() { 0.0 } else { unit(v) }
+}
+
 /// `c` with its components in range.
 pub(crate) fn clamp_color(c: Color) -> Color {
     match c {
@@ -589,8 +594,8 @@ pub(crate) fn clamp_color(c: Color) -> Color {
     }
 }
 
-/// A paint from plug-in output (`null`: none), with colours, opacities and offsets clamped and
-/// gradient and pattern placements checked.
+/// A paint from plug-in output (`null`: none), with colors, process plates, opacities and
+/// offsets clamped and gradient and pattern placements checked.
 fn decode_paint(v: &Value, at: &str) -> Result<Paint> {
     if v.is_null() {
         return Ok(Paint::None);
@@ -613,6 +618,9 @@ fn decode_paint(v: &Value, at: &str) -> Result<Paint> {
                 s.midpoint = s.midpoint.clamp(0.13, 0.87);
                 s.tint = unit(s.tint);
                 s.color = clamp_color(s.color);
+                if let Some(plates) = &mut s.plates {
+                    *plates = plates.map(plate);
+                }
             }
             stops.sort_by(|a, b| a.offset.total_cmp(&b.offset));
             g.angle = g.angle.rem_euclid(360.0);