Disable layout-2013 feature by default (#34290)

Most of the time layout-2013 is not needed and just
wastes build time. Disable the optional feature by
default and require users to specify the feature
at compile time if they wish to use the legacy layout.

In CI we enable the feature by default for Linux, since
that we need it for wpt tests with the legacy layout
and CI should ensure that the legacy code continues
to compile.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

.github/workflows/linux.yml

@@ -152,7 +152,7 @@ jobs:
 
       - name: Build (${{ inputs.profile }})
         run: |
-          python3 ./mach build --use-crown --locked --${{ inputs.profile }}
+          python3 ./mach build --use-crown --locked --${{ inputs.profile }} --features "layout_2013"
           cp -r target/cargo-timings target/cargo-timings-linux
       - name: Smoketest
         run: xvfb-run python3 ./mach smoketest --${{ inputs.profile  }}

components/servo/lib.rs

@@ -1033,8 +1033,8 @@ fn get_layout_factory(legacy_layout: bool) -> Arc<dyn LayoutFactory> {
             }
         } else {
             if legacy_layout {
-                warn!("Runtime option `legacy_layout` was enabled, but the `layout_2013` \
-                feature was not enabled at compile time. Falling back to layout 2020! ");
+                panic!("Runtime option `legacy_layout` was enabled, but the `layout_2013` \
+                feature was not enabled at compile time! ");
            }
         }
     }

ports/servoshell/Cargo.toml

@@ -39,7 +39,7 @@ ProductName = "Servo"
 
 [features]
 debugmozjs = ["libservo/debugmozjs"]
-default = ["layout_2013", "max_log_level", "webdriver", "webxr"]
+default = ["max_log_level", "webdriver", "webxr"]
 jitspew = ["libservo/jitspew"]
 js_backtrace = ["libservo/js_backtrace"]
 layout_2013 = ["libservo/layout_2013"]