[dxvk] Reduced staging buffer size

Allocating as much memory as we did was wasteful. The new limit may
still be too generous for most applications, but this will require
more testing.

src/dxvk/dxvk_device.h

@@ -33,7 +33,7 @@ namespace dxvk {
   class DxvkDevice : public RcObject {
     friend class DxvkSubmissionQueue;
     
-    constexpr static VkDeviceSize DefaultStagingBufferSize = 64 * 1024 * 1024;
+    constexpr static VkDeviceSize DefaultStagingBufferSize = 16 * 1024 * 1024;
   public:
     
     DxvkDevice(