Update Windows, macOS, and AppImage dependencies

- SDL -> 3.4.14
- FFmpeg -> 9.0
- libplacebo -> 4d82c689

.github/workflows/build-appimage.yml

@@ -42,7 +42,7 @@ jobs:
         uses: actions/checkout@v5
         with:
           repository: libsdl-org/SDL
-          ref: c9ad296376ed6091e0fdec9844461fdf09af7845
+          ref: release-3.4.14
           path: deps/SDL
 
       - name: Build SDL3
@@ -101,7 +101,7 @@ jobs:
         uses: actions/checkout@v5
         with:
           repository: haasn/libplacebo
-          ref: 2d0979fb54e025e904c7372666fffbf5dae40f66
+          ref: 4d82c6898551068d4ae6a6b5538efcddc2c7cf64
           path: deps/libplacebo
           submodules: 'recursive'
 
@@ -129,7 +129,7 @@ jobs:
         uses: actions/checkout@v5
         with:
           repository: FFmpeg/FFmpeg
-          ref: n8.1.1
+          ref: n9.0
           path: deps/FFmpeg
 
       - name: Build FFmpeg