ICO image format: fix regression in writing when size >= 256

In commit c6c93041

, the earlier size limit of 128 was raised to the
format's defined maximum of 256. But the required special storage of
this size in the image structures was not implemented. Hence,
attempting to store such big icons would result in invalid image
files.

Fix the size storing details, and add some autotests of ico format
writing since that was practically uncovered.

Task-number: QTBUG-53259
Change-Id: I00e17a04e90c32dcf1124ba5adaf53728fb74dc7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>