[dxvk] Increase update buffer size

Helps reduce the number of memory allocations further
when deferred contexts are used for rendering.

src/d3d11/d3d11_context.cpp

@@ -3007,7 +3007,7 @@ namespace dxvk {
 
 
   DxvkDataSlice D3D11DeviceContext::AllocUpdateBufferSlice(size_t Size) {
-    constexpr size_t UpdateBufferSize = 4 * 1024 * 1024;
+    constexpr size_t UpdateBufferSize = 16 * 1024 * 1024;
 
     if (Size >= UpdateBufferSize) {
       Rc<DxvkDataBuffer> buffer = new DxvkDataBuffer(Size);