mouse: Make pointer warp emulation via relative mode available to all platforms

Move the Wayland pointer warp emulation code up to the SDL mouse layer, and activate it when a client attempts to warp a hidden mouse cursor when the hint is set.

testrelative adds the ability to test the warp emulation activation/deactivation with the --warp parameter and 'c' key for toggling cursor visibility.

test/testrelative.c

@@ -12,6 +12,7 @@
 
 /* Simple program:  Test relative mouse motion */
 
+#include <SDL3/SDL_test.h>
 #include <SDL3/SDL_test_common.h>
 #include <SDL3/SDL_main.h>
 
@@ -21,16 +22,47 @@
 
 static SDLTest_CommonState *state;
 static int i, done;
-static float mouseX, mouseY;
 static SDL_FRect rect;
 static SDL_Event event;
+static SDL_bool warp;
 
 static void DrawRects(SDL_Renderer *renderer)
 {
     SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
-    rect.x = mouseX;
-    rect.y = mouseY;
     SDL_RenderFillRect(renderer, &rect);
+
+    SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
+    if (SDL_GetRelativeMouseMode()) {
+        SDLTest_DrawString(renderer, 0.f, 0.f, "Relative Mode: Enabled");
+    } else {
+        SDLTest_DrawString(renderer, 0.f, 0.f, "Relative Mode: Disabled");
+    }
+}
+
+static void CenterMouse()
+{
+    /* Warp the mouse back to the center of the window with input focus to use the
+     * center point for calculating future motion deltas.
+     *
+     * NOTE: DO NOT DO THIS IN REAL APPS/GAMES!
+     *
+     *       This is an outdated method of handling relative pointer motion, and
+     *       may not work properly, if at all, on some platforms. It is here *only*
+     *       for testing the warp emulation code path internal to SDL.
+     *
+     *       Relative mouse mode should be used instead!
+     */
+    SDL_Window *window = SDL_GetKeyboardFocus();
+    if (window) {
+        int w, h;
+        float cx, cy;
+
+        SDL_GetWindowSize(window, &w, &h);
+        cx = (float)w / 2.f;
+        cy = (float)h / 2.f;
+
+        SDL_WarpMouseInWindow(window, cx, cy);
+    }
 }
 
 static void loop(void)
@@ -39,21 +71,46 @@ static void loop(void)
     while (SDL_PollEvent(&event)) {
         SDLTest_CommonEvent(state, &event, &done);
         switch (event.type) {
+        case SDL_EVENT_WINDOW_FOCUS_GAINED:
+            if (warp) {
+                /* This should activate relative mode for warp emulation, unless disabled via a hint. */
+                CenterMouse();
+            }
+            break;
+        case SDL_EVENT_KEY_DOWN:
+            if (event.key.key == SDLK_C) {
+                /* If warp emulation is active, showing the cursor should turn
+                 * relative mode off, and it should re-activate after a warp
+                 * when hidden again.
+                 */
+                if (SDL_CursorVisible()) {
+                    SDL_HideCursor();
+                } else {
+                    SDL_ShowCursor();
+                }
+            }
+            break;
         case SDL_EVENT_MOUSE_MOTION:
         {
-            mouseX += event.motion.xrel;
-            mouseY += event.motion.yrel;
+            rect.x += event.motion.xrel;
+            rect.y += event.motion.yrel;
+
+            if (warp) {
+                CenterMouse();
+            }
         } break;
         default:
             break;
         }
     }
+
     for (i = 0; i < state->num_windows; ++i) {
         SDL_Rect viewport;
         SDL_Renderer *renderer = state->renderers[i];
         if (state->windows[i] == NULL) {
             continue;
         }
+
         SDL_SetRenderDrawColor(renderer, 0x00, 0x00, 0x00, 0xFF);
         SDL_RenderClear(renderer);
 
@@ -85,7 +142,6 @@ static void loop(void)
 
 int main(int argc, char *argv[])
 {
-
     /* Enable standard application logging */
     SDL_SetLogPriority(SDL_LOG_CATEGORY_APPLICATION, SDL_LOG_PRIORITY_INFO);
 
@@ -96,8 +152,27 @@ int main(int argc, char *argv[])
     }
 
     /* Parse commandline */
-    if (!SDLTest_CommonDefaultArgs(state, argc, argv)) {
-        return 1;
+    for (i = 1; i < argc;) {
+        int consumed;
+
+        consumed = SDLTest_CommonArg(state, i);
+        if (consumed == 0) {
+            consumed = -1;
+            if (SDL_strcasecmp(argv[i], "--warp") == 0) {
+                warp = SDL_TRUE;
+                consumed = 1;
+            }
+        }
+
+        if (consumed < 0) {
+            static const char *options[] = {
+                "[--warp]",
+                NULL
+            };
+            SDLTest_CommonLogUsage(state, argv[0], options);
+            return 1;
+        }
+        i += consumed;
     }
 
     if (!SDLTest_CommonInit(state)) {
@@ -112,8 +187,17 @@ int main(int argc, char *argv[])
         SDL_RenderClear(renderer);
     }
 
-    if (SDL_SetRelativeMouseMode(SDL_TRUE) < 0) {
-        return 3;
+    /* If warp mode is activated, the cursor will be repeatedly warped back to
+     * the center of the window to simulate the behavior of older games. The cursor
+     * is initially hidden in this case to trigger the warp emulation unless it has
+     * been explicitly disabled via a hint.
+     *
+     * Otherwise, try to activate relative mode.
+     */
+    if (warp) {
+        SDL_HideCursor();
+    } else if (SDL_SetRelativeMouseMode(SDL_TRUE) < 0) {
+        return 3; /* Relative mode failed, just exit. */
     }
 
     rect.x = DEFAULT_WINDOW_WIDTH / 2;