d3d12: reset the vertex buffer size when it is released

(cherry-picked from commit 490c20f93f)

src/render/direct3d12/SDL_render_d3d12.c

@@ -352,6 +352,7 @@ D3D12_ReleaseAll(SDL_Renderer * renderer)
 
         for (i = 0; i < SDL_D3D12_NUM_VERTEX_BUFFERS; ++i) {
             SAFE_RELEASE(data->vertexBuffers[i].resource);
+            data->vertexBuffers[i].size = 0;
         }
         
         data->swapEffect = (DXGI_SWAP_EFFECT) 0;