This implement horizontal scrolling of rows to allow editing insets
(math, tabular...) that are larger then the screen. The scrolling happens
as the cursor moves, in order to make sure that the cursor is always visible.
This effectively closes an 11 years old bug.
This feature is the result of the work of Hashini Senaratne as part of
Google Summer of Code 2013. The code has been cleaned-up for inclusion
and remaining bugs have been fixed.
Fixes bug: #1083.
All the code that is run before row metrics have been computed should use int arithmetic. After metrics have been computed, we still need doubles because fully justified rows use double for Row::Element::extra.
Rename Row::x to Row::left_margin and change its type to int.
Rename Row::Element::width() to full_width(). In some places of the code, use dim.wid (the int version without the extra separator) because metrics have not been computed.
Let Row::Element::x2pos take a int& argument instead of double&
Let Row::Element::breakAt take a int argument instead of double
