Added support for external mouse in Samsung DeX mode

relative mode doesn't work, but absolute coordinates are functional

src/core/android/SDL_android.c

@@ -215,6 +215,7 @@ static jmethodID midSetOrientation;
 static jmethodID midGetContext;
 static jmethodID midIsAndroidTV;
 static jmethodID midIsChromebook;
+static jmethodID midIsDeXMode;
 static jmethodID midInputGetInputDeviceIds;
 static jmethodID midSendMessage;
 static jmethodID midShowTextInput;
@@ -320,6 +321,8 @@ JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeSetupJNI)(JNIEnv* mEnv, jclass c
                                 "isAndroidTV","()Z");
     midIsChromebook = (*mEnv)->GetStaticMethodID(mEnv, mActivityClass,
                                 "isChromebook", "()Z");
+    midIsDeXMode = (*mEnv)->GetStaticMethodID(mEnv, mActivityClass,
+                                "isDeXMode", "()Z");
     midInputGetInputDeviceIds = (*mEnv)->GetStaticMethodID(mEnv, mActivityClass,
                                 "inputGetInputDeviceIds", "(I)[I");
     midSendMessage = (*mEnv)->GetStaticMethodID(mEnv, mActivityClass,
@@ -354,7 +357,7 @@ JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeSetupJNI)(JNIEnv* mEnv, jclass c
        !midClipboardSetText || !midClipboardGetText || !midClipboardHasText ||
        !midOpenAPKExpansionInputStream || !midGetManifestEnvironmentVariables || !midGetDisplayDPI ||
        !midCreateCustomCursor || !midSetCustomCursor || !midSetSystemCursor || !midSupportsRelativeMouse || !midSetRelativeMouseEnabled ||
-       !midIsChromebook) {
+       !midIsChromebook || !midIsDeXMode) {
         __android_log_print(ANDROID_LOG_WARN, "SDL", "Missing some Java callbacks, do you have the latest version of SDLActivity.java?");
     }
 
@@ -2036,6 +2039,12 @@ SDL_bool SDL_IsChromebook(void)
     return (*env)->CallStaticBooleanMethod(env, mActivityClass, midIsChromebook);
 }
 
+SDL_bool SDL_IsDeXMode(void)
+{
+    JNIEnv *env = Android_JNI_GetEnv();
+    return (*env)->CallStaticBooleanMethod(env, mActivityClass, midIsDeXMode);
+}
+
 const char * SDL_AndroidGetInternalStoragePath(void)
 {
     static char *s_AndroidInternalFilesPath = NULL;

src/dynapi/SDL_dynapi_overrides.h

@@ -673,3 +673,4 @@
 #define SDL_LinuxSetThreadPriority SDL_LinuxSetThreadPriority_REAL
 #define SDL_HasAVX512F SDL_HasAVX512F_REAL
 #define SDL_IsChromebook SDL_IsChromebook_REAL
+#define SDL_IsDeXMode SDL_IsDeXMode_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -714,4 +714,5 @@ SDL_DYNAPI_PROC(int,SDL_LinuxSetThreadPriority,(Sint64 a, int b),(a,b),return)
 SDL_DYNAPI_PROC(SDL_bool,SDL_HasAVX512F,(void),(),return)
 #ifdef __ANDROID__
 SDL_DYNAPI_PROC(SDL_bool,SDL_IsChromebook,(void),(),return)
+SDL_DYNAPI_PROC(SDL_bool,SDL_IsDeXMode,(void),(),return)
 #endif

src/events/SDL_mouse.c

@@ -721,6 +721,9 @@ SDL_SetRelativeMouseMode(SDL_bool enabled)
     } else if (mouse->SetRelativeMouseMode(enabled) < 0) {
         if (enabled) {
             /* Fall back to warp mode if native relative mode failed */
+            if (!mouse->WarpMouse) {
+                return SDL_SetError("No relative mode implementation available");
+            }
             mouse->relative_mode_warp = SDL_TRUE;
         }
     }