[dxvk] Fixed staging buffer alignment

Four-byte alignment is required in order to avoid illegal
vkCmdCopyBufferToImage calls. We align staging buffer slices to
a full cache line in order to improve performance.

src/dxvk/dxvk_staging.cpp

@@ -35,7 +35,7 @@ namespace dxvk {
     slice.offset = m_bufferOffset;
     slice.mapPtr = m_buffer->mapPtr(m_bufferOffset);
     
-    m_bufferOffset += size;
+    m_bufferOffset = align(m_bufferOffset + size, 64);
     return true;
   }