Remove unnecessary ClipDisplayItems in box_.rs

[pradeep90]

For ScannedTextBox and ImageBox, the ClipDisplayItem's child_list is
currently always empty.

ClipDisplayItem is used to implement overflow hidden and should only be
created for block containers, as per
http://www.w3.org/TR/CSS2/visufx.html#propdef-overflow

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/665

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Warning

• These commits modify layout code, but no reftests are modified. Please consider adding a reftest!
• @pradeep90, please confirm that src/test/html/acid1.html and your favourite wikipedia page still render correctly!

[pradeep90]

Right now, it is failing for some edge case ('fail to find parent item').

I will fix it and get back here.

[pradeep90]

Fixed the bug where an img with display: block (no children) was failing to find the parent item for the empty children display_list.

[pradeep90]

Rebased on latest Servo code.

r? @larsbergstrom

[pradeep90]

@larsbergstrom The FIXME is not needed any more because that part is contained within if self.is_block_container().

I think the only case when InlineFlow will have children flows is when inline-blocks are supported. Then, they will have a GenericBox and thus will generate a ClipDisplayItemClass in which the children display items will go.

[pradeep90]

@larsbergstrom I don't think that was considered as an r+ for the PR. Nothing has happened :)