[librsvg] surface_utils: pass down opacity as a UnitInterval

[Paolo Borelli <pborelli src gnome org>]

commit d1cc13a2688a08d7b8542293f32fece0309a4a6a
Author: Paolo Borelli <pborelli gnome org>
Date:   Mon Jan 6 12:10:33 2020 +0100

    surface_utils: pass down opacity as a UnitInterval
    
    The u8 representation is an internal detail of cairo.

 rsvg_internals/src/drawing_ctx.rs                  | 2 +-
 rsvg_internals/src/surface_utils/shared_surface.rs | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/rsvg_internals/src/drawing_ctx.rs b/rsvg_internals/src/drawing_ctx.rs
index c3528646..a90af76a 100644
--- a/rsvg_internals/src/drawing_ctx.rs
+++ b/rsvg_internals/src/drawing_ctx.rs
@@ -478,7 +478,7 @@ impl DrawingCtx {
         let Opacity(opacity) = values.opacity;
 
         let mask = SharedImageSurface::new(mask_content_surface, SurfaceType::SRgb)?
-            .to_mask(u8::from(opacity))?
+            .to_mask(opacity)?
             .into_image_surface()?;
 
         Ok(Some(mask))
diff --git a/rsvg_internals/src/surface_utils/shared_surface.rs 
b/rsvg_internals/src/surface_utils/shared_surface.rs
index 069b07df..f96c5fcf 100644
--- a/rsvg_internals/src/surface_utils/shared_surface.rs
+++ b/rsvg_internals/src/surface_utils/shared_surface.rs
@@ -10,6 +10,7 @@ use nalgebra::{storage::Storage, Dim, Matrix};
 
 use crate::rect::IRect;
 use crate::srgb;
+use crate::unit_interval::UnitInterval;
 use crate::util::clamp;
 
 use super::{
@@ -413,7 +414,7 @@ impl SharedImageSurface {
     /// useful luminance data.
     ///
     /// This is to get a mask suitable for use with cairo_mask_surface().
-    pub fn to_mask(&self, opacity: u8) -> Result<SharedImageSurface, cairo::Status> {
+    pub fn to_mask(&self, opacity: UnitInterval) -> Result<SharedImageSurface, cairo::Status> {
         let bounds = IRect::from_size(self.width, self.height);
 
         let mut output_surface =
@@ -422,6 +423,7 @@ impl SharedImageSurface {
         let stride = output_surface.get_stride() as usize;
         {
             let mut data = output_surface.get_data().unwrap();
+            let opacity = u8::from(opacity);
 
             for (x, y, pixel) in Pixels::new(self, bounds) {
                 data.set_pixel(stride, pixel.to_mask(opacity), x, y);