theme: Pre-generate stylesheets as well

Since commit 9ef9a5ff8a, man pages are generated at dist time to reduce
build dependencies when building from a released tarball.

Do the same for the stylesheets to also remove the sassc dependency.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1555>

meson/generate-stylesheets.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python3
+
+import os
+from pathlib import PurePath
+import subprocess
+
+stylesheets = [
+    'data/theme/gnome-shell-high-contrast.css',
+    'data/theme/gnome-shell.css'
+]
+
+sourceroot = os.environ.get('MESON_SOURCE_ROOT')
+distroot = os.environ.get('MESON_DIST_ROOT')
+
+for stylesheet in stylesheets:
+    stylesheet_path = PurePath(stylesheet)
+    src = PurePath(sourceroot, stylesheet_path.with_suffix('.scss'))
+    dst = PurePath(distroot, stylesheet_path)
+    subprocess.call(['sassc', '-a', src, dst])