Add dependency on python3-gi-cairo everywhere it seems to matter.

Resolves #4857.

It seems to me that Debian requires an extra package for PyCairo, but
RPM and PIP based installs do not. That's strange, but at least this
much seems to be needed.

.travis.yml

@@ -18,6 +18,7 @@ addons:
       - libdbus-1-dev
       - python3-yaml
       - python3-gi
+      - python3-gi-cairo
       - python3-pil
       - python3-setproctitle
       - python3-distro

INSTALL.rst

@@ -12,7 +12,7 @@ Lutris manually, it requires the following components:
 
     * Python >= 3.7
     * PyGObject
-    * PyGObject bindings for: Gtk, Gdk, GnomeDesktop, Webkit2, Notify
+    * PyGObject bindings for: Gtk, Gdk, Cairo, GnomeDesktop, Webkit2, Notify
     * python3-requests
     * python3-pillow
     * python3-yaml
@@ -40,7 +40,7 @@ games themselves we recommend you install the following packages:
 To install all those dependencies (except for Wine and graphics drivers)
 on Ubuntu based systems, you can run::
 
-    sudo apt install python3-yaml python3-requests python3-pil python3-gi \
+    sudo apt install python3-yaml python3-requests python3-pil python3-gi python3-gi-cairo \
       gir1.2-gtk-3.0 gir1.2-gnomedesktop-3.0 gir1.2-webkit2-4.0 \
       gir1.2-notify-0.7 psmisc cabextract unzip p7zip curl fluid-soundfont-gs \
       x11-xserver-utils python3-evdev libc6-i386 lib32gcc1 libgirepository1.0-dev \

debian/control

@@ -21,6 +21,7 @@ Depends: ${misc:Depends},
          python3-requests,
          python3-pil,
          python3-gi,
+         python3-gi-cairo,
          python3-setproctitle,
          python3-magic,
          python3-distro,