At its highest level, redisplay can be divided into 3 distinct
steps, all of which are visible in `redisplay_internal':
- . decide which windows on which frames need their windows
- considered for redisplay
+ . decide which frames need their windows to be considered for redisplay
. for each window whose display might need to be updated, compute
a structure, called "glyph matrix", which describes how it
should look on display