metadata: Report if image can be transparent

This feature was not implemented before and would always return true,
triggering a line detection for the background color in large JPEG
images.

Part-of: <https://gitlab.gnome.org/GNOME/loupe/-/merge_requests/442>

src/decoder/formats/glycin_proxy.rs

@@ -69,7 +69,7 @@ impl Glycin {
             let frame = image.next_frame().await?;
 
             let mut metadata: Metadata = Metadata::default();
-            metadata.set_frame_info(frame.details().clone());
+            metadata.set_frame_metadata(&frame);
             updater.send(DecoderUpdate::Metadata(Box::new(metadata)));
             tiles.set_original_dimensions((frame.width() as u32, frame.height() as u32));
 

src/metadata.rs

@@ -25,6 +25,7 @@ mod file;
 mod gps;
 
 pub use file::FileInfo;
+use glycin::{Frame, MemoryFormat};
 use glycin::{FrameDetails, ImageInfoDetails};
 pub use gps::GPSLocation;
 
@@ -39,6 +40,7 @@ pub struct Metadata {
     file_info: Option<FileInfo>,
     image_info: Option<glycin::ImageInfoDetails>,
     frame_info: Option<FrameDetails>,
+    memory_format: Option<MemoryFormat>,
 }
 
 impl std::fmt::Debug for Metadata {
@@ -80,8 +82,9 @@ impl Metadata {
         self.image_info = Some(image_info);
     }
 
-    pub fn set_frame_info(&mut self, frame_info: FrameDetails) {
-        self.frame_info = Some(frame_info);
+    pub fn set_frame_metadata(&mut self, frame: &Frame) {
+        self.frame_info = Some(frame.details().clone());
+        self.memory_format = Some(frame.memory_format());
     }
 
     pub fn set_file_info(&mut self, file_info: FileInfo) {
@@ -144,8 +147,7 @@ impl Metadata {
     }
 
     pub fn is_potentially_transparent(&self) -> bool {
-        // TODO: Implement again
-        true
+        self.memory_format.is_some_and(|x| x.has_alpha())
     }
 
     pub fn transformations_applied(&self) -> bool {