Make sure Mac and Linux builds have SDL2 available.

Currently Windows gets this from vcpkg. Mac will get it from Homebrew,
and the GitHub linux builders will just install it with apt-get.

This might not be the perfect solution (having to install Homebrew is a pain,
GitHub Actions has an ancient SDL2), but it gets all the common platforms
running for now without much fuss.

.github/workflows/build.yml

@@ -67,7 +67,7 @@ jobs:
         run: |
           sudo apt update
           sudo apt install -y --no-install-recommends \
-            ninja-build cmake g++ libgtest-dev libsdl1.2-dev libsdl-image1.2-dev libncurses-dev zlib1g-dev
+            ninja-build cmake g++ libgtest-dev libsdl2-dev libncurses-dev zlib1g-dev
 
       - name: Configure CMake
         env:

Brewfile

@@ -1,8 +1,7 @@
 # Homebrew dependencies to build Descent3
 
 # SDL
-brew "sdl12-compat"
-brew "sdl2_image"
+brew "sdl2"
 
 brew "cmake"
 brew "googletest"