Support svg buttons

In the theme directory add close-{active,inactive}.svg instead of
close.xbm - and similarly for iconify, menu and max.

.github/workflows/build.yml

@@ -67,7 +67,7 @@ jobs:
           pacman-key --init
           pacman -Syu --noconfirm
           pacman -S --noconfirm git meson clang wlroots libdrm libinput \
-            wayland-protocols cairo pango libxml2 xorg-xwayland
+            wayland-protocols cairo pango libxml2 xorg-xwayland librsvg
 
       - name: Install Debian Testing dependencies
         if: matrix.name == 'Debian'
@@ -77,7 +77,8 @@ jobs:
           apt-get upgrade -y
           apt-get install -y git clang \
             hwdata \
-            libxml2-dev libcairo2-dev libpango1.0-dev
+            libxml2-dev libcairo2-dev libpango1.0-dev \
+            librsvg2-dev
           apt-get build-dep -y wlroots
 
       - name: Install FreeBSD dependencies
@@ -108,6 +109,7 @@ jobs:
             xcb-util-cursor-devel xcb-util-devel xcb-util-image-devel \
             xcb-util-keysyms-devel xcb-util-xrm-devel xorg-server-xwayland \
             hwids \
+            librsvg-devel \
             libglib-devel cairo-devel pango-devel
 
       - name: Build with gcc