gpu renderer: fixed memory leak when resizing the backbuffer

Fixes https://github.com/libsdl-org/SDL/issues/14734

src/render/gpu/SDL_render_gpu.c

@@ -1457,6 +1457,8 @@ static bool CreateBackbuffer(GPU_RenderData *data, Uint32 w, Uint32 h, SDL_GPUTe
     tci.sample_count = SDL_GPU_SAMPLECOUNT_1;
     tci.usage = SDL_GPU_TEXTUREUSAGE_COLOR_TARGET | SDL_GPU_TEXTUREUSAGE_SAMPLER;
 
+    SDL_ReleaseGPUTexture(data->device, data->backbuffer.texture);
+
     data->backbuffer.texture = SDL_CreateGPUTexture(data->device, &tci);
     data->backbuffer.width = w;
     data->backbuffer.height = h;