Auto merge of #13848 - mrobinson:remove-layers, r=glennw

Remove concept of Layers from Servo

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because this PR should not change behavior.

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Layers were a feature of the legacy drawing path. If we re-add them at
some point, it probably makes more sense to make them a product of
display list inspection.

This change also remove a bunch of dead painting code.

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components/compositing/compositor.rs

@@ -12,7 +12,7 @@ use euclid::point::TypedPoint2D;
 use euclid::scale_factor::ScaleFactor;
 use euclid::size::TypedSize2D;
 use gfx_traits::{DevicePixel, LayerPixel, StackingContextId};
-use gfx_traits::{Epoch, FrameTreeId, FragmentType, LayerId};
+use gfx_traits::{Epoch, FrameTreeId, FragmentType};
 use gleam::gl;
 use gleam::gl::types::{GLint, GLsizei};
 use image::{DynamicImage, ImageFormat, RgbImage};
@@ -523,9 +523,9 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 self.title_for_main_frame();
             }
 
-            (Msg::ScrollFragmentPoint(pipeline_id, layer_id, point, _),
+            (Msg::ScrollFragmentPoint(pipeline_id, point, _),
              ShutdownState::NotShuttingDown) => {
-                self.scroll_fragment_to_point(pipeline_id, layer_id, point);
+                self.scroll_fragment_to_point(pipeline_id, point);
             }
 
             (Msg::MoveTo(point),
@@ -796,7 +796,6 @@ impl<Window: WindowMethods> IOCompositor<Window> {
 
     fn scroll_fragment_to_point(&mut self,
                                 _pipeline_id: PipelineId,
-                                _layer_id: LayerId,
                                 _point: Point2D<f32>) {
         println!("TODO: Support scroll_fragment_to_point again");
     }

components/compositing/compositor_thread.rs

@@ -8,7 +8,6 @@ use SendableFrameTree;
 use compositor::CompositingReason;
 use euclid::point::Point2D;
 use euclid::size::Size2D;
-use gfx_traits::LayerId;
 use ipc_channel::ipc::IpcSender;
 use msg::constellation_msg::{Image, Key, KeyModifiers, KeyState, PipelineId};
 use profile_traits::mem;
@@ -72,7 +71,7 @@ pub enum Msg {
     ShutdownComplete,
 
     /// Scroll a page in a window
-    ScrollFragmentPoint(PipelineId, LayerId, Point2D<f32>, bool),
+    ScrollFragmentPoint(PipelineId, Point2D<f32>, bool),
     /// Alerts the compositor that the current page has changed its title.
     ChangePageTitle(PipelineId, Option<String>),
     /// Alerts the compositor that the current page has changed its URL.

components/compositing/scrolling.md

@@ -1,57 +0,0 @@
-Scrolling
-=========
-
-Scrolling is implemented by the compositor. Compositor layers that opt in to
-scroll events via the `wants_scroll_events` flag can scroll their contents.
-These will be referred "scrolling roots." Scrolling roots serve as a viewport
-into their content, which is stored in descendant layers. In order for
-scrolling roots to be able to scroll their content, they need to be smaller
-than that content. If the content was smaller than the scrolling root, it would
-not be able to move around inside the scrolling root. Imagine a browser window
-that is larger than the content that it contains. The size of each layer is
-defined by the window size (the root layer) or the block size for iframes and
-elements with `overflow:scroll`.
-
-Since the compositor allows layers to exist independently of their parents,
-child layers can overflow or fail to intersect their parents completely. To
-prevent this, scrolling roots use the `masks_to_bounds` flag, which is a signal
-to the compositor that it should not paint the parts of descendant layers that
-lie outside the boundaries of the scrolling root.
-
-Below is an ASCII art diagram showing a scrolling root with three content
-layers (a, b, and c), scrolled down a few ticks. `masks_to_bounds` has not been
-applied in the diagram.
-
-<pre>
-+-----------------------+
-|                       |
-=========================
-|                       |  scrolling
-|           &lt;-------------+root
-|                       |
-|             +-------+ |
-=========================
-|             |  b    | |
-++-------+    +--^----+ |
-||       |       |      |
-||       |       |      |  content
-||   c &lt;---------+---------+layers
-|+-------+    /         |
-|          a &lt;          |
-|                       |
-+-----------------------+
-</pre>
-
-Everything above and below the set of `====` bars would be hidden by
-`masks_to_bounds`, so the composited scene will just be the viewport defined by
-the scrolling root with the content layers a and b visible.
-
-<pre>
-=========================
-|                       |
-|                       |
-|                       |
-|             +-------+ |
-=========================
-</pre>
-