swf: Fix blur scaling in filters

The blur of 1.0 is invariant, so that it should be preserved when scaling.
Using `blur * scale` also scales the invariant, which is wrong.
Tests show that `(blur - 1) * scale + 1` matches FP's behavior.

swf/src/types/bevel_filter.rs

@@ -35,8 +35,8 @@ impl BevelFilter {
     }
 
     pub fn scale(&mut self, x: f32, y: f32) {
-        self.blur_x *= Fixed16::from_f32(x);
-        self.blur_y *= Fixed16::from_f32(y);
+        self.blur_x = BlurFilter::scale_blur(self.blur_x, x);
+        self.blur_y = BlurFilter::scale_blur(self.blur_y, y);
         self.distance *= Fixed16::from_f32(y);
     }
 

swf/src/types/blur_filter.rs

@@ -23,8 +23,8 @@ impl BlurFilter {
     }
 
     pub fn scale(&mut self, x: f32, y: f32) {
-        self.blur_x *= Fixed16::from_f32(x);
-        self.blur_y *= Fixed16::from_f32(y);
+        self.blur_x = Self::scale_blur(self.blur_x, x);
+        self.blur_y = Self::scale_blur(self.blur_y, y);
     }
 
     pub fn impotent(&self) -> bool {
@@ -42,6 +42,11 @@ impl BlurFilter {
             y_max: source_rect.y_max + y,
         }
     }
+
+    #[inline]
+    pub(crate) fn scale_blur(blur: Fixed16, factor: f32) -> Fixed16 {
+        (blur - Fixed16::ONE) * Fixed16::from_f32(factor) + Fixed16::ONE
+    }
 }
 
 bitflags! {

swf/src/types/drop_shadow_filter.rs

@@ -37,8 +37,8 @@ impl DropShadowFilter {
     }
 
     pub fn scale(&mut self, x: f32, y: f32) {
-        self.blur_x *= Fixed16::from_f32(x);
-        self.blur_y *= Fixed16::from_f32(y);
+        self.blur_x = BlurFilter::scale_blur(self.blur_x, x);
+        self.blur_y = BlurFilter::scale_blur(self.blur_y, y);
         self.distance *= Fixed16::from_f32(y);
     }
 

swf/src/types/glow_filter.rs

@@ -32,8 +32,8 @@ impl GlowFilter {
     }
 
     pub fn scale(&mut self, x: f32, y: f32) {
-        self.blur_x *= Fixed16::from_f32(x);
-        self.blur_y *= Fixed16::from_f32(y);
+        self.blur_x = BlurFilter::scale_blur(self.blur_x, x);
+        self.blur_y = BlurFilter::scale_blur(self.blur_y, y);
     }
 
     pub fn calculate_dest_rect(&self, source_rect: Rectangle<Twips>) -> Rectangle<Twips> {

swf/src/types/gradient_filter.rs

@@ -34,8 +34,8 @@ impl GradientFilter {
     }
 
     pub fn scale(&mut self, x: f32, y: f32) {
-        self.blur_x *= Fixed16::from_f32(x);
-        self.blur_y *= Fixed16::from_f32(y);
+        self.blur_x = BlurFilter::scale_blur(self.blur_x, x);
+        self.blur_y = BlurFilter::scale_blur(self.blur_y, y);
         self.distance *= Fixed16::from_f32(y);
     }