Really release pool resources in Renderer::releaseCachedResources

resize(0) only sets the size and doesn't deallocate anything.
Conversely, shrink(0), which was used before it was changed
to resize(0) doesn't change the size and only deallocates.
Use a mix of both for now.

Pick-to: 6.5
Change-Id: I3e36a766ef7483158a5a300351b0d7864d24e184
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/quick/scenegraph/coreapi/qsgbatchrenderer.cpp

@@ -960,8 +960,10 @@ void Renderer::releaseCachedResources()
 
     m_rhi->releaseCachedResources();
 
-    m_vertexUploadPool.resize(0);
-    m_indexUploadPool.resize(0);
+    m_vertexUploadPool.shrink(0);
+    m_vertexUploadPool.reset();
+    m_indexUploadPool.shrink(0);
+    m_indexUploadPool.reset();
 }
 
 void Renderer::invalidateAndRecycleBatch(Batch *b)