Replace FontMetrics::breakAt, which returned the next break point,
with FontMetrics::breakString, which returns a vector of break points.
To this end, an additional parameter gives the available width for
next rows.
Rename various variables and methods accordingly. Factor the code in
breakString_helper to be more manageable.
Adapt Row::Element::splitAt to return a bool on sucess and provide
remaining row elements in a vector. The width noted above has been
added as parameters.
Rename the helper function splitFrom to moveElements and rewrite the
code to be more efficient.
Remove type of row element INVALID, which is not needed anymore.
The code in TextMetrics::breakParagraph is now much simpler.
In Row::finalize, remove the code that computed inconditionnally the
current element size, and make sure that this width will be computed
in all code paths of Row::Element::splitAt.
Since we intend to break the row element in two, it is not good to
truncate the string too early.
Moreover, the row element width is now set at this point, even if no
breaking occurs.
This fixes two performance issues and improves the performance of
TextMetrics::redoParagraph by 15% in a workload that uses the cache a
lot. The difference will be much less when the cache is not used much.
1/ repetion of the hash code computation
The code
if (cache.contains(key))
result = cache[key]:
is not efficient, since qHash(key) has to be computed twice.
To fix this a new Cache::object_str() method is added, which allows
if (auto * obj = cache.object(key))
result = *obj;
2/ code of has code computation
Instead of using a verbose string that is complicated to build as
key, new key structs BreakAtKey and TextLayoutKey are introduced,
along with the relevant qHash() implementation.
In particular, the directory frontends/qt4 is renamed to frontends/qt.
Many configurations file have to be updated. All mentions of qt4 in
the source have been audited, and changed to qt if necessary.
The only part that has not been updated is the CMake build system.
