Replace Git submodule with copies of modules

This is to avoid needing to rely on a Git submodule to build this
project. Instead, the equivalent files from these modules have been
placed in this repository.

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "platforms/flatpak/shared-modules"]
-	path = platforms/flatpak/shared-modules
-	url = https://github.com/flathub/shared-modules.git

platforms/flatpak/io.github.drhelius.Gearsystem.yml

@@ -10,9 +10,53 @@ finish-args:
   - --socket=pulseaudio
   - --device=all # Allows for controller input
 modules:
-  - shared-modules/glu/glu-9.json
-  - shared-modules/glew/glew.json
-  - shared-modules/SDL2/SDL2-no-libdecor.json
+  - name: glu
+    buildsystem: meson
+    sources:
+      - type: archive
+        url: https://archive.mesa3d.org/glu/glu-9.0.3.tar.xz
+        sha256: bd43fe12f374b1192eb15fe20e45ff456b9bc26ab57f0eee919f96ca0f8a330f
+  - name: glew
+    no-autogen: true
+    make-args:
+      - GLEW_PREFIX=${FLATPAK_DEST}
+      - GLEW_DEST=${FLATPAK_DEST}
+      - LIBDIR=${FLATPAK_DEST}/lib
+      - CFLAGS.EXTRA:=${CFLAGS} -fPIC
+      - LDFLAGS.EXTRA=${LDFLAGS}
+    make-install-args:
+      - GLEW_PREFIX=${FLATPAK_DEST}
+      - GLEW_DEST=${FLATPAK_DEST}
+      - LIBDIR=${FLATPAK_DEST}/lib
+      - CFLAGS.EXTRA:=${CFLAGS} -fPIC
+      - LDFLAGS.EXTRA=${LDFLAGS}
+    sources:
+      - type: archive
+        url: https://downloads.sourceforge.net/project/glew/glew/2.2.0/glew-2.2.0.tgz
+        sha256: d4fc82893cfb00109578d0a1a2337fb8ca335b3ceccf97b97e5cc7f08e4353e1
+    cleanup:
+      - /include
+      - /lib/pkgconfig
+      - /lib/*.a
+  - name: SDL2
+    buildsystem: autotools
+    config-opts:
+      - --disable-static
+    sources:
+      - type: archive
+        url: https://github.com/libsdl-org/SDL/releases/download/release-2.30.0/SDL2-2.30.0.tar.gz
+        sha256: 36e2e41557e0fa4a1519315c0f5958a87ccb27e25c51776beb6f1239526447b0
+    cleanup:
+      - /bin/sdl2-config
+      - /include
+      - /lib/libSDL2.la
+      - /lib/libSDL2main.a
+      - /lib/libSDL2main.la
+      - /lib/libSDL2_test.a
+      - /lib/libSDL2_test.la
+      - /lib/cmake
+      - /share/aclocal
+      - /lib/pkgconfig
   - name: nfde
     buildsystem: cmake
     config-opts: