The strategy adopted in bc47054b had some drawbacks related to the way
instant preview snippets are generated. See the subthread starting at
http://www.mail-archive.com/lyx-devel@lists.lyx.org/msg187916.html
for details.
The strategy adopted in this commit is that of adding macro definitions
only for the macros actually used in a preview snippet, independently
of whether some macro was already used in a previous snippet. In this way
the snippets don't need to be changed according to whether they are
compiled as a whole or separately from each other. This fact was causing
the regeneration of a preview snippet whenever the cursor entered the
corresponding inset, even if the generated image would have not changed.
The problem of defining or redefining a macro is taken care by the
python scripts.
11c2b7792c