SDL_ClaimWindowForGPUDevice() should fail for transparent windows

The GPU API doesn't currently support transparent windows (transparent swapchain effects doesn't seem possible on D3D12) so we should explicitly fail so users don't expect transparency and then not get it.

Fixes https://github.com/libsdl-org/SDL/issues/12410

(cherry picked from commit 2fbb583290dfbb547f23daaa09e6cb3cf08900cb)

src/gpu/SDL_gpu.c

@@ -2646,8 +2646,11 @@ bool SDL_ClaimWindowForGPUDevice(
 {
     CHECK_DEVICE_MAGIC(device, false);
     if (window == NULL) {
-        SDL_InvalidParamError("window");
-        return false;
+        return SDL_InvalidParamError("window");
+    }
+
+    if ((window->flags & SDL_WINDOW_TRANSPARENT) != 0) {
+        return SDL_SetError("The GPU API doesn't support transparent windows");
     }
 
     return device->ClaimWindow(