[librsvg: 7/14] Add an alpha-only flag to SharedImageSurface

[Federico Mena Quintero <federico src gnome org>]

commit cfc20af4626b4cf2f264fe266f43c238efb0852b
Author: Ivan Molodetskikh <yalterz gmail com>
Date:   Sun Jul 15 13:57:27 2018 +0300

    Add an alpha-only flag to SharedImageSurface

 rsvg_internals/src/surface_utils/shared_surface.rs | 28 +++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)
---
diff --git a/rsvg_internals/src/surface_utils/shared_surface.rs 
b/rsvg_internals/src/surface_utils/shared_surface.rs
index f4ffd632..23cc45be 100644
--- a/rsvg_internals/src/surface_utils/shared_surface.rs
+++ b/rsvg_internals/src/surface_utils/shared_surface.rs
@@ -46,6 +46,12 @@ pub struct SharedImageSurface {
     width: i32,
     height: i32,
     stride: isize,
+
+    /// Whether this surface contains meaningful data only in the alpha channel.
+    ///
+    /// This is used for optimizations, particularly in `convolve()` to skip processing other
+    /// channels.
+    alpha_only: bool,
 }
 
 impl SharedImageSurface {
@@ -82,9 +88,23 @@ impl SharedImageSurface {
             width,
             height,
             stride,
+            alpha_only: false,
         })
     }
 
+    /// Creates a `SharedImageSurface` from a unique `ImageSurface` with meaningful data only in
+    /// the alpha channel.
+    ///
+    /// # Panics
+    /// Panics if the surface format isn't `ARgb32` and if the surface is not unique, that is, its
+    /// reference count isn't 1.
+    #[inline]
+    pub fn new_alpha_only(surface: ImageSurface) -> Result<Self, cairo::Status> {
+        let mut rv = Self::new(surface)?;
+        rv.alpha_only = true;
+        Ok(rv)
+    }
+
     /// Converts this `SharedImageSurface` back into a Cairo image surface.
     ///
     /// # Panics
@@ -111,6 +131,12 @@ impl SharedImageSurface {
         self.height
     }
 
+    /// Returns `true` if the surface contains meaningful data only in the alpha channel.
+    #[inline]
+    pub fn is_alpha_only(&self) -> bool {
+        self.alpha_only
+    }
+
     /// Retrieves the pixel value at the given coordinates.
     #[inline]
     pub fn get_pixel(&self, x: u32, y: u32) -> Pixel {
@@ -226,7 +252,7 @@ impl SharedImageSurface {
             }
         }
 
-        SharedImageSurface::new(output_surface)
+        SharedImageSurface::new_alpha_only(output_surface)
     }
 
     /// Returns a surface with pre-multiplication of color values undone.