lyx_mirror/00README_STR_METRICS_BRANCH

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This branch is where I (jmarc) try to implement string_wise metrics
computation. This is done through a series of cleanups. The expected
speed improvement will only be visible at the end of the road: indeed
for now we intend to keep unchanged behavior for testing purposes.
What is done:
* Make TextMetrics methods operate on Row objects: breakRow and
setRowHeight instead of rowBreakPoint and rowHeight.
* change breakRow operation to operate on text strings on which
metrics are computed. The list of elements is stored in the row object
* Implement proper string metrics computation (with cache), when
lyxrc.force_paint_single_char is false.
Next steps:
* get rid of rowWidth (breakRow does compute this)
* re-implement getColumnNearX using row elements
* re-implement cursorX using row elements
* re-implement row painting using row elements (can it be done?)
* profile and see how performance can be improved.
Difference in behavior
* words longer than the screen are no monger broken at an arbitrary
point. This will not be useful anymore with horizontal scrolling.
* end of paragraph markers metrics are computed with the font of the
actual text, not default font. This will be extended to the other
methods.
Other differences that should be considered as bugs
* there are still some difference in width computation wrt
TextMetrics::rowWidth. This happens in particular with Description
environment when the row is broken at bodypos. The method rowWidth
is kept for now in order to be able to detect row parsing errors,
but it could be removed right now.