Add initial support for offscreen rendering (#30767)

* Offscreen rendering

* shared memory case never actually rendered to backbuffer

* fix compile errors (in theory) when gl crate feature disabled

* update doc comments

* remove dark CentralPanel border covering edges of viewport

* clear to transparent, to avoid pink artifacts

* fix mouse input for browser being consumed by egui

* avoid destroying OpenGL resources unless resizing window

* clean up compositing::gl

* fix flickering around edges after resizing window

* unset invalidate_last_render_target after invalidating

* fix incorrect DRAW_FRAMEBUFFER name when blitting

* bind the widget surface fbo before painting egui

* make composite_specific_target take CompositeTarget, not Option

* compositing: remove cargo feature “gl”

* capitalise FBO in bind log message

Co-authored-by: Martin Robinson <mrobinson@igalia.com>

* capitalise FBO in drop log message

Co-authored-by: Martin Robinson <mrobinson@igalia.com>

* rename RenderTargetInfo fields and use OnceCell for next field

* rename RenderTargetInfo.read to read_back_from_gpu

* document servo_framebuffer_id in Minibrowser::update

* rename needs_fbo to use_offscreen_framebuffer

* capitalise FBO in unbind log message

* clarify the purpose of Minibrowser::on_event

* fix unused_must_use warning

* reduce nesting in Minibrowser::update

* use implicit format argument in panic

* store Minibrowser.widget_surface_fbo as glow type

* explain why servo_framebuffer_id is None in first call site

* rename output_framebuffer_id to offscreen_framebuffer_id

---------

Co-authored-by: Martin Robinson <mrobinson@igalia.com>

2 files changed, 16 insertions, 3 deletions

diff --git a/components/servo/Cargo.toml b/components/servo/Cargo.toml
index e3bdc4921a4..e7a6e786534 100644
--- a/components/servo/Cargo.toml
+++ b/components/servo/Cargo.toml
@@ -37,7 +37,7 @@ bluetooth = { path = "../bluetooth" }
 bluetooth_traits = { workspace = true }
 canvas = { path = "../canvas", default-features = false }
 canvas_traits = { workspace = true }
-compositing = { path = "../compositing", features = ["gl"] }
+compositing = { path = "../compositing" }
 compositing_traits = { workspace = true }
 constellation = { path = "../constellation" }
 crossbeam-channel = { workspace = true }
diff --git a/components/servo/lib.rs b/components/servo/lib.rs
index 122ff4c6d2d..424f3fc647e 100644
--- a/components/servo/lib.rs
+++ b/components/servo/lib.rs
@@ -30,7 +30,7 @@ use canvas::canvas_paint_thread::{self, CanvasPaintThread};
 use canvas::WebGLComm;
 use canvas_traits::webgl::WebGLThreads;
 use compositing::windowing::{EmbedderEvent, EmbedderMethods, WindowMethods};
-use compositing::{IOCompositor, InitialCompositorState, ShutdownState};
+use compositing::{CompositeTarget, IOCompositor, InitialCompositorState, ShutdownState};
 use compositing_traits::{
     CanvasToCompositorMsg, CompositingReason, CompositorMsg, CompositorProxy, CompositorReceiver,
     ConstellationMsg, FontToCompositorMsg, ForwardedToCompositorMsg,
@@ -224,6 +224,7 @@ where
         mut embedder: Box<dyn EmbedderMethods>,
         window: Rc<Window>,
         user_agent: Option<String>,
+        composite_target: CompositeTarget,
     ) -> InitializedServo<Window> {
         // Global configuration options, parsed from the command line.
         let opts = opts::get();
@@ -447,6 +448,12 @@ where
             }
         }
 
+        let composite_target = if let Some(path) = opts.output_file.clone() {
+            CompositeTarget::PngFile(path.into())
+        } else {
+            composite_target
+        };
+
         // The compositor coordinates with the client window to create the final
         // rendered page and display it somewhere.
         let compositor = IOCompositor::create(
@@ -464,7 +471,7 @@ where
                 webrender_gl,
                 webxr_main_thread,
             },
-            opts.output_file.clone(),
+            composite_target,
             opts.is_running_problem_test,
             opts.exit_after_load,
             opts.debug.convert_mouse_to_touch,
@@ -769,6 +776,12 @@ where
     pub fn recomposite(&mut self) {
         self.compositor.composite();
     }
+
+    /// Return the OpenGL framebuffer name of the most-recently-completed frame when compositing to
+    /// [`CompositeTarget::Fbo`], or None otherwise.
+    pub fn offscreen_framebuffer_id(&self) -> Option<u32> {
+        self.compositor.offscreen_framebuffer_id()
+    }
 }
 
 fn create_embedder_channel(