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.
