cd ~/RPIDoom3Installer/SDLAddOns
sudo dpkg -i sdl2-image_2.0.5-1_armhf.deb
sudo dpkg -i sdl2-mixer_2.0.4-1_armhf.deb
sudo dpkg -i sdl2-ttf_2.0.15-1_armhf.deb
installing these packages is very dangerous for the system as it breaks any of the official sdl2 packages from installing, since you changed the names of these packages and include both dev libraries and non-dev libraries in one package
these default packages should work just fine..
libsdl2-dev libsdl2-image-2.0-0
libsdl2-image-dev libsdl2-mixer-2.0-0 libsdl2-mixer-dev libsdl2-ttf-2.0-0
libsdl2-ttf-dev
there should be no reason to install the custom packages from here, as all the libraries are contained in the official packages. why did you do so?