The QSGBatchRenderer::Renderer's m_vertexUploadPool and m_indexUploadPool buffers never shrink

If QtQuick application dynamically allocates memory for QML objects and then deallocates the same objects with destroy() only part of the memory is returned back to the OS.

Steps to reproduce.

1. Please run the attached example. 
2. Inspect the memory usage of the application process (valgrind --tool=massif --pages-as-heap=yes, pmap -d or top)
3. Create QML objects by pressing the button in the application
4. Repeat step 2.)
5. Delete the QML objects by pressing the button in the application
6. Repeat step 2.)

It can be observed that only about 1/3 of the memory that was allocated is returned back to the OS.

mapped: 753432K writeable/private: 85488K shared: 18652K
mapped: 1112792K writeable/private: 437268K shared: 52304K
mapped: 1040892K writeable/private: 365752K shared: 52304K