webgpu: implement get image for webgpu canvas (#35237)

* Implement `get_image_data` for WebGPU canvas

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

* Update expectations

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

* Add docs

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/webgpu/ipc_messages/recv.rs

@@ -154,6 +154,11 @@ pub enum WebGPURequest {
         texture_id: id::TextureId,
         encoder_id: id::CommandEncoderId,
     },
+    /// Obtains image from latest presentation buffer (same as wr update)
+    GetImage {
+        context_id: WebGPUContextId,
+        sender: IpcSender<IpcSharedMemory>,
+    },
     ValidateTextureDescriptor {
         device_id: id::DeviceId,
         texture_id: id::TextureId,

components/webgpu/swapchain.rs

@@ -9,7 +9,7 @@ use std::sync::{Arc, Mutex};
 
 use arrayvec::ArrayVec;
 use euclid::default::Size2D;
-use ipc_channel::ipc::IpcSender;
+use ipc_channel::ipc::{IpcSender, IpcSharedMemory};
 use log::{error, warn};
 use malloc_size_of::MallocSizeOf;
 use serde::{Deserialize, Serialize};
@@ -382,6 +382,22 @@ impl crate::WGPU {
         );
     }
 
+    pub(crate) fn get_image(&self, context_id: WebGPUContextId) -> IpcSharedMemory {
+        let webgpu_contexts = self.wgpu_image_map.lock().unwrap();
+        let context_data = webgpu_contexts.get(&context_id).unwrap();
+        let buffer_size = context_data.image_desc.buffer_size();
+        let data = if let Some(present_buffer) = context_data
+            .swap_chain
+            .as_ref()
+            .and_then(|swap_chain| swap_chain.data.as_ref())
+        {
+            IpcSharedMemory::from_bytes(present_buffer.slice())
+        } else {
+            IpcSharedMemory::from_byte(0, buffer_size as usize)
+        };
+        data
+    }
+
     pub(crate) fn update_context(
         &self,
         context_id: WebGPUContextId,

components/webgpu/wgpu_thread.rs

@@ -531,6 +531,9 @@ impl WGPU {
                             log::error!("Error occured in SwapChainPresent: {e:?}");
                         }
                     },
+                    WebGPURequest::GetImage { context_id, sender } => {
+                        sender.send(self.get_image(context_id)).unwrap()
+                    },
                     WebGPURequest::ValidateTextureDescriptor {
                         device_id,
                         texture_id,