pixman: Store strong reference to WlBuffer in PixmanImage

If a weak `WlBuffer` is used, it isn't possible to read the contents of
a destroyed shm buffer. But the Wayland protocol specifies that
destroying an attached buffer shouldn't change the contents of the
surface.

A `wayland_server::Weak` will not upgrade to a strong reference for a
destroyed object even if other strong references exist, like the strong
reference to the `WlBuffer` in `WaylandSurfaceRenderElement`.

This fixes an error when a buffer is destroyed by libdecor, which
crashed a version of Anvil patched to use pixman.

src/backend/renderer/pixman/mod.rs

@@ -24,7 +24,7 @@ use crate::{
     wayland::{compositor::SurfaceData, shm},
 };
 #[cfg(feature = "wayland_frontend")]
-use wayland_server::{protocol::wl_buffer, Resource, Weak};
+use wayland_server::protocol::wl_buffer;
 
 #[cfg(all(
     feature = "wayland_frontend",
@@ -88,7 +88,7 @@ struct PixmanDmabufMapping {
 #[derive(Debug)]
 struct PixmanImageInner {
     #[cfg(feature = "wayland_frontend")]
-    buffer: Option<Weak<wl_buffer::WlBuffer>>,
+    buffer: Option<wl_buffer::WlBuffer>,
     dmabuf: Option<PixmanDmabufMapping>,
     image: Mutex<Image<'static, 'static>>,
     _flipped: bool, /* TODO: What about flipped textures? */
@@ -102,7 +102,7 @@ impl PixmanImage {
     fn accessor<'l>(&'l self) -> Result<TextureAccessor<'l>, PixmanError> {
         #[cfg(feature = "wayland_frontend")]
         let buffer = if let Some(buffer) = self.0.buffer.as_ref() {
-            Some(buffer.upgrade().map_err(|_| PixmanError::BufferDestroyed)?)
+            Some(buffer)
         } else {
             None
         };
@@ -116,7 +116,7 @@ impl PixmanImage {
 
         Ok(TextureAccessor {
             #[cfg(feature = "wayland_frontend")]
-            buffer,
+            buffer: buffer.cloned(),
             image: &self.0.image,
             _guard: guard,
         })
@@ -1124,7 +1124,7 @@ impl ImportMemWl for PixmanRenderer {
             std::result::Result::<_, PixmanError>::Ok(image)
         })??;
         Ok(PixmanTexture(PixmanImage(Arc::new(PixmanImageInner {
-            buffer: Some(buffer.downgrade()),
+            buffer: Some(buffer.clone()),
             dmabuf: None,
             image: Mutex::new(image),
             _flipped: false,