CI Linux: do not build own SDL2 anymore

SDL2 now obviously works on U20.04, because we are building there. It
doesn't work on U22.04, but neither the build with SDL 2.0.20 nor 2.26.2
does.

The problem seems actually to be the inclusion of Wayland libraries in
AppImage.

.github/scripts/Linux/download_build_sdl.sh

@@ -1,30 +0,0 @@
-#!/bin/sh -eu
-
-mkdir -p /var/tmp/sdl
-cd /var/tmp/sdl
-
-# v2.0.22 requires wayland-client version 1.18.0 but in U18.04 is only 1.16.0
-curl -sSLO https://github.com/libsdl-org/SDL/releases/download/release-2.0.20/SDL2-2.0.20.tar.gz
-tar xaf SDL2*tar*
-cd SDL2-2.0.20
-./configure
-make -j "$(nproc)"
-sudo make install # SDL needs to be installed here because it is a dependency for the below
-cd ..
-
-git clone -b SDL2 --depth 1 https://github.com/libsdl-org/SDL_mixer
-cd SDL_mixer
-./configure
-make -j "$(nproc)"
-sudo make install
-cd ..
-
-# v2.0.18 and further require automake 1.16 but U18.04 has only automake 1.15.1
-git clone -b release-2.0.15 --depth 1 https://github.com/libsdl-org/SDL_ttf
-cd SDL_ttf
-./autogen.sh # to allow automake 1.15.1
-./configure
-make -j "$(nproc)"
-sudo make install
-cd ..
-

.github/scripts/Linux/install_sdl.sh

@@ -1,9 +0,0 @@
-#!/bin/sh -eu
-
-cd /var/tmp/sdl/SDL2-2.0.20
-sudo make install
-cd ../SDL_mixer
-sudo make install
-cd ../SDL_ttf
-sudo make install
-

.github/scripts/Linux/prepare.sh

@@ -19,6 +19,7 @@ sudo apt --no-install-recommends install nvidia-cuda-toolkit
 sudo apt install libglew-dev libglfw3-dev
 sudo apt install libglm-dev
 sudo apt install libx11-dev
+sudo apt install libsdl2-dev libsdl2-mixer-dev libsdl2-ttf-dev
 sudo apt install libsoxr-dev libspeexdsp-dev
 sudo apt install libssl-dev
 sudo apt install libasound-dev libjack-jackd2-dev libnatpmp-dev libv4l-dev portaudio19-dev