build: Make sassc mandatory and always generate CSS files from Sass sources

The previous implementation of the CSS generation logic considered sassc
an optional dependency and made sure for that reason that the result CSS
files ended up in the source directory, so that they could be checked
in and kept in sync with the source sass files.

As we are making sassc a required dependency, we can now stop doing that
extra work and simply rely on the CSS files being automatically generated
each time the sass sources change. By doing this, we can now effectively
get rid of the CSS files checked in the repo as well as of the parse_sass.sh
script, since the CSS files will now live on the build directory only.

https://bugzilla.gnome.org/show_bug.cgi?id=792822

data/theme/README

@@ -1,12 +1,9 @@
 Summary
 -------
 
-* Do not edit the CSS directly, edit the source SCSS files and process them with SASS (running
-  `make` should do that when you have the required software installed, as described below;
-  run `./parse-sass.sh` manually if it doesn't)
-* Most SASS preprocessors should produce similar results, however the build system
-  integration and 'parse-sass.sh' script use sassc. You should be able to install
-  it with `pkcon install sassc` or your distribution's package manager.
+* Do not edit the CSS directly, edit the source SCSS files and the CSS files will be generated
+  automatically when building with meson + ninja and left inside the build directory to be
+  incorporated into the gresource XML (you'll need to have sassc installed).
 
 How to tweak the theme
 ----------------------
@@ -31,4 +28,4 @@ _common.scss        - actual definitions of style for each widget. This is where
                       your changes.
                       
 You can read about SASS at http://sass-lang.com/documentation/. Once you make your changes to the
-_common.scss file, you can either run make or the ./parse-sass.sh script.
+_common.scss file, you can run ninja to generate the final CSS files.

data/theme/meson.build

@@ -7,15 +7,21 @@ theme_sources = files([
   'gnome-shell-sass/_high-contrast-colors.scss'
 ])
 
+styles = [
+  'gnome-shell-high-contrast',
+  'gnome-shell'
+]
+
 theme_deps = []
 
 if sassc.found()
-  parse_sass = files('parse-sass.sh')
-
-  theme_deps += custom_target('update-theme',
-    output: 'theme-update.stamp',
-    depend_files: theme_sources,
-    command: [parse_sass, '@OUTPUT@'],
-    build_by_default: true
-  )
+  foreach style: styles
+    theme_deps += custom_target('style-' + style,
+                                input: '@0@.scss'.format(style),
+                                output: '@0@.css'.format(style),
+                                command: [
+                                  sassc, '-a', '@INPUT@', '@OUTPUT@'
+                                ],
+                                depend_files: theme_sources)
+  endforeach
 endif

data/theme/parse-sass.sh

@@ -1,10 +0,0 @@
-#!/bin/sh
-
-srcdir=`dirname $0`
-stamp=${1}
-for scss in $srcdir/*.scss
-do
-  sassc -a $scss ${scss%%.scss}.css || exit 1
-done
-
-[ "$stamp" ] && touch $stamp

meson.build

@@ -138,7 +138,7 @@ endif
 
 mutter_typelibdir = mutter_dep.get_pkgconfig_variable('typelibdir')
 python = find_program('python3')
-sassc = find_program('sassc', required: false)
+sassc = find_program('sassc', required: true)
 
 cc = meson.get_compiler('c')