Fix FBO recreated every time the QSG node is updated under some conditions

When textureFollowsItemsSize is true and the item width/height are (1)
not precise integer values or (2) less than the minimum FBO
width/height, then QQuickFramebufferObject will delete and recreate the
FBO every time the QSG node is updated. This patch fixes the issue.

Task-number: QTBUG-40548
Change-Id: I532aaaa88a5c604ee7cc1fd8f0acb0601c5a94fb
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>