Improve memory usage of QQuickTextNode.

The typical number of BinaryTreeNodes needed for a text line should be
much lower than 256.

Task-number: QTBUG-32770
Change-Id: I85aa161eb7cb6e55657213304b7577a0a33f1b67
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>