[dxvk] Reduce context staging buffer size to 4 MiB

Same idea as before, just create a temporary buffer for larger resources.

This can avoid frequent Vulkan memory allocations and deallocations since
many small buffers are more likely to fit into a single memory chunk than
a small number of large buffers, thus reducing the overall memory footprint.

src/dxvk/dxvk_context.h

@@ -19,7 +19,7 @@ namespace dxvk {
    * recorded.
    */
   class DxvkContext : public RcObject {
-    constexpr static VkDeviceSize StagingBufferSize = 32ull << 20;
+    constexpr static VkDeviceSize StagingBufferSize = 4ull << 20;
   public:
     
     DxvkContext(const Rc<DxvkDevice>& device);