SDL3 update (#787)

* Update to SDL3

* Add XTest development dependency for SDL3

* Preserve native entry points with SDL3

* Document SDL3 XTest dependency

.github/workflows/build.yml

@@ -314,6 +314,7 @@ jobs:
             libxkbcommon-dev \
             libxrandr-dev \
             libxss-dev \
+            libxtst-dev \
             lld \
             pkg-config \
             wayland-protocols
@@ -359,7 +360,7 @@ jobs:
             -DCMAKE_PREFIX_PATH="$Qt6_DIR" 2>&1 | tee _Build/configure.log
           exit "${PIPESTATUS[0]}"
 
-      - name: Verify SDL2 backends
+      - name: Verify SDL3 backends
         shell: bash
         run: |
           status=0