CI Linux: build fluidsynth alongside with SDL

In U22.04 (current Linux CI image), fluidsynth depends on sdl2, which
is undesirable. Actually we need to avoid sdl2 linking altogether.

Build from sources alongside with SDL. This makes sense since SDL_mixer
used to be build there and fluidsynth is replacing it.

+ prevent libsdl2-2.0-0 from (accidentally) installing

Author: MartinPulec

.github/scripts/Linux/download_build_sdl.sh

@@ -17,3 +17,9 @@ cmake --build build -j "$(nproc)"
 sudo cmake --install build
 cd ..
 
+git clone --recurse-submodules --depth 1\
+ https://github.com/Fluidsynth/fluidsynth
+cmake -S fluidsynth -B fluidsynth/build
+cmake --build fluidsynth/build -j "$(nproc)"
+sudo cmake --install fluidsynth/build
+

.github/scripts/Linux/install_sdl.sh

@@ -1,7 +1,7 @@
 #!/bin/sh -eu
 
-cd /var/tmp/sdl/SDL
-sudo cmake --install build
-cd ../SDL_ttf
-sudo cmake --install build
+cd /var/tmp/sdl
+sudo cmake --install SDL/build
+sudo cmake --install SDL_ttf/build
+sudo cmake --install fluidsynth/build
 

.github/scripts/Linux/prepare.sh

@@ -15,12 +15,12 @@ else # one-line-style (old) format
         sed -n '/^deb /s/^deb /deb-src /p' /etc/apt/sources.list |
                 sudo tee /etc/apt/sources.list.d/sources.list
 fi
+sudo apt-mark hold libsdl2-2.0-0
 sudo apt update
 sudo apt install appstream `# appstreamcli for mkappimage AppStream validation` \
         asciidoc
 sudo apt install fonts-dejavu-core
 sudo apt --no-install-recommends install nvidia-cuda-toolkit
-sudo apt install libfluidsynth-dev
 sudo apt install libglew-dev libglfw3-dev
 sudo apt install libglm-dev
 sudo apt install imagemagick libmagickwand-dev
@@ -36,9 +36,10 @@ get_build_deps_excl() { # $2 - pattern to exclude; separate packates with '\|' (
 }
 
 sudo apt build-dep libsdl2
+fluidsynth_build_dep=$(get_build_deps_excl libfluidsynth3 libsdl2-dev)
 sdl2_ttf_build_dep=$(get_build_deps_excl libsdl2-ttf libsdl2-dev)
 # shellcheck disable=SC2086 # intentional
-sudo apt install $sdl2_ttf_build_dep
+sudo apt install $fluidsynth_build_dep $sdl2_ttf_build_dep
 
 ffmpeg_build_dep=$(get_build_deps_excl ffmpeg 'libsdl')
 # shellcheck disable=SC2086 # intentional