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>

components/compositing/lib.rs

@@ -13,10 +13,9 @@ use webrender::RenderApi;
 use webrender_api::DocumentId;
 use webrender_surfman::WebrenderSurfman;
 
-pub use crate::compositor::{IOCompositor, ShutdownState};
+pub use crate::compositor::{CompositeTarget, IOCompositor, ShutdownState};
 
 mod compositor;
-#[cfg(feature = "gl")]
 mod gl;
 mod touch;
 pub mod windowing;