fix(svg): disable rsvg by default

When SVG support is enabled via librsvg, launching builds of Emacs 27.1
or later from the Finder into GUI mode will crash Emacs if the fancy
startup screen is used. This crash has been observed with librsvg
2.46.4, 2.48.7 and 2.48.8.

Hence SVG image support is now disabled by default. Should you need it,
you will need to disable the startup screen to avoid Emacs crashing, by
putting this in your init.el:

    (setq inhibit-startup-screen t)

The cause of the issue seems to be the `image-size` function as called
from `use-fancy-splash-screens-p`.

README.md

@@ -77,6 +77,7 @@ Options:
         --[no-]native-comp-macos-fixes
                                      Enable/disable fix based on feature/native-comp-macos-fixes branch (default: enabled if native-comp supported)
         --[no-]launcher              Enable/disable embedded launcher script  (default: enabled if native-comp is enabled)
+        --rsvg                       Enable SVG image support via librsvg, can yield a unstable build (default: disabled)
         --no-titlebar                Apply no-titlebar patch (default: disabled)
         --no-frame-refocus           Apply no-frame-refocus patch (default: disabled)
 ```

build-emacs-for-macos

@@ -306,6 +306,7 @@ class Build
         configure_flags << '--with-xwidgets'
       end
       configure_flags << '--with-nativecomp' if options[:native_comp]
+      configure_flags << '--without-rsvg' unless options[:rsvg]
 
       run_cmd './configure', *configure_flags
 
@@ -793,10 +794,11 @@ end
 
 if __FILE__ == $PROGRAM_NAME
   cli_options = {
+    macos_fixes: true,
     native_fast_boot: true,
     parallel: Etc.nprocessors,
-    xwidgets: true,
-    macos_fixes: true
+    rsvg: false,
+    xwidgets: true
   }
 
   OptionParser.new do |opts|
@@ -850,6 +852,11 @@ if __FILE__ == $PROGRAM_NAME
       cli_options[:launcher] = v
     end
 
+    opts.on('--rsvg', 'Enable SVG image support via librsvg, ' \
+                      'can yield a unstable build (default: disabled)') do
+      cli_options[:rsvg] = true
+    end
+
     opts.on('--no-titlebar', 'Apply no-titlebar patch (default: disabled)') do
       cli_options[:no_titlebar] = true
     end