Remove xcore graphics backend

This removes X11 specific code, making SDL2 a hard dependency.
X-based environments are continue to be supported via SDL2.

This also removes joystick_linux.c which was only used if SDL2 was not
available.

.github/workflows/linux.yml

@@ -15,7 +15,7 @@ jobs:
     - name: Install Deps
       run: |
         sudo apt update
-        sudo apt install libsdl2-dev libsdl2-ttf-dev libsdl2-mixer-dev
+        sudo apt install libgtk2.0-dev libsdl2-dev libsdl2-ttf-dev libsdl2-mixer-dev
 
     - name: Build
       run: |
@@ -27,26 +27,3 @@ jobs:
     - name: Test
       run: ctest --output-on-failure
       working-directory: out/${{ matrix.build-type }}
-
-  build-x11:
-    runs-on: ubuntu-latest
-
-    name: Linux X11
-    steps:
-    - uses: actions/checkout@v2
-
-    - name: Install Deps
-      run: |
-        sudo apt update
-        sudo apt install libgtk2.0-dev
-
-    - name: Build
-      run: |
-        mkdir -p out
-        cd out
-        cmake ../
-        make
-
-    - name: Test
-      run: ctest --output-on-failure
-      working-directory: out