Auto merge of #27766 - jdm:embedding-blit, r=asajeffrey

Embedding support for offscreen-rendering

The current embedding API assumes it should render directly to a native widget at all times. These changes expose an option to render to an offscreen surface when starting the engine, as well as a new WebrenderSurfman API for temporarily acquiring  the surface corresponding to the front buffer so it can be used for blitting purposes.

Demonstrated to work in https://github.com/jdm/servo-embedding-example.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

components/webrender_surfman/lib.rs

@@ -162,6 +162,23 @@ impl WebrenderSurfman {
             })
     }
 
+    /// Invoke a closure with the surface associated with the current front buffer.
+    /// This can be used to create a surfman::SurfaceTexture to blit elsewhere.
+    pub fn with_front_buffer<F: FnMut(&Device, Surface) -> Surface>(&self, mut f: F) {
+        let ref mut device = self.0.device.borrow_mut();
+        let ref mut context = self.0.context.borrow_mut();
+        let surface = device
+            .unbind_surface_from_context(context)
+            .unwrap()
+            .unwrap();
+        let surface = f(device, surface);
+        device.bind_surface_to_context(context, surface).unwrap();
+    }
+
+    pub fn device(&self) -> std::cell::Ref<Device> {
+        self.0.device.borrow()
+    }
+
     pub fn connection(&self) -> Connection {
         let ref device = self.0.device.borrow();
         device.connection()

ports/libsimpleservo/api/src/lib.rs

@@ -63,10 +63,18 @@ pub struct InitOptions {
     pub xr_discovery: Option<webxr::Discovery>,
     pub gl_context_pointer: Option<*const c_void>,
     pub native_display_pointer: Option<*const c_void>,
-    pub native_widget: *mut c_void,
+    pub surfman_integration: SurfmanIntegration,
     pub prefs: Option<HashMap<String, PrefValue>>,
 }
 
+/// Controls how this embedding's rendering will integrate with the embedder.
+pub enum SurfmanIntegration {
+    /// Render directly to a provided native widget (see surfman::NativeWidget).
+    Widget(*mut c_void),
+    /// Render to an offscreen surface.
+    Surface,
+}
+
 #[derive(Clone, Debug)]
 pub struct Coordinates {
     pub viewport: Rect<i32, DevicePixel>,
@@ -153,6 +161,7 @@ pub trait HostTrait {
 }
 
 pub struct ServoGlue {
+    webrender_surfman: WebrenderSurfman,
     servo: Servo<ServoWindowCallbacks>,
     batch_mode: bool,
     callbacks: Rc<ServoWindowCallbacks>,
@@ -253,13 +262,21 @@ pub fn init(
             .create_adapter()
             .or(Err("Failed to create adapter"))?,
     };
+    let surface_type = match init_opts.surfman_integration {
+        SurfmanIntegration::Widget(native_widget) => {
             let native_widget = unsafe {
                 connection.create_native_widget_from_ptr(
-            init_opts.native_widget,
+                    native_widget,
                     init_opts.coordinates.framebuffer.to_untyped(),
                 )
             };
-    let surface_type = SurfaceType::Widget { native_widget };
+            SurfaceType::Widget { native_widget }
+        },
+        SurfmanIntegration::Surface => {
+            let size = init_opts.coordinates.framebuffer.to_untyped();
+            SurfaceType::Generic { size }
+        },
+    };
     let webrender_surfman = WebrenderSurfman::create(&connection, &adapter, surface_type)
         .or(Err("Failed to create surface manager"))?;
 
@@ -269,7 +286,7 @@ pub fn init(
         density: init_opts.density,
         gl_context_pointer: init_opts.gl_context_pointer,
         native_display_pointer: init_opts.native_display_pointer,
-        webrender_surfman,
+        webrender_surfman: webrender_surfman.clone(),
     });
 
     let embedder_callbacks = Box::new(ServoEmbedderCallbacks {
@@ -282,6 +299,7 @@ pub fn init(
 
     SERVO.with(|s| {
         let mut servo_glue = ServoGlue {
+            webrender_surfman,
             servo,
             batch_mode: false,
             callbacks: window_callbacks,
@@ -322,6 +340,12 @@ impl ServoGlue {
         self.servo.deinit();
     }
 
+    /// Returns the webrender surface management integration interface.
+    /// This provides the embedder access to the current front buffer.
+    pub fn surfman(&self) -> WebrenderSurfman {
+        self.webrender_surfman.clone()
+    }
+
     /// This is the Servo heartbeat. This needs to be called
     /// everytime wakeup is called or when embedder wants Servo
     /// to act on its pending events.

ports/libsimpleservo/capi/src/lib.rs

@@ -21,6 +21,7 @@ use simpleservo::{self, gl_glue, ServoGlue, SERVO};
 use simpleservo::{
     ContextMenuResult, Coordinates, DeviceIntRect, EventLoopWaker, HostTrait, InitOptions,
     InputMethodType, MediaSessionActionType, MediaSessionPlaybackState, MouseButton, PromptResult,
+    SurfmanIntegration,
 };
 use std::ffi::{CStr, CString};
 #[cfg(target_os = "windows")]
@@ -463,7 +464,7 @@ unsafe fn init(
         xr_discovery: None,
         gl_context_pointer: gl_context,
         native_display_pointer: display,
-        native_widget: opts.native_widget,
+        surfman_integration: SurfmanIntegration::Widget(opts.native_widget),
     };
 
     let wakeup = Box::new(WakeupCallback::new(wakeup));