Variable: reference-point-alist

Alist of symbols vs integer codes of glyph reference points.
A glyph reference point symbol is to be used to specify a composition
rule in COMPONENTS argument to such functions as `compose-region'.

The meaning of glyph reference point codes is as follows:

0----1----2 <---- ascent 0:tl or top-left
| | 1:tc or top-center
| | 2:tr or top-right
| | 3:Bl or base-left 9:cl or center-left
9 10 11 <---- center 4:Bc or base-center 10:cc or center-center
| | 5:Br or base-right 11:cr or center-right
--3----4----5-- <-- baseline 6:bl or bottom-left
| | 7:bc or bottom-center
6----7----8 <---- descent 8:br or bottom-right

Glyph reference point symbols are to be used to specify composition
rule of the form (GLOBAL-REF-POINT . NEW-REF-POINT), where
GLOBAL-REF-POINT is a reference point in the overall glyphs already
composed, and NEW-REF-POINT is a reference point in the new glyph to
be added.

For instance, if GLOBAL-REF-POINT is `br' (bottom-right) and
NEW-REF-POINT is `tc' (top-center), the overall glyph is updated as
follows (the point `*' corresponds to both reference points):

+-------+--+ <--- new ascent
| | |
| global| |
| glyph | |
-- | | |-- <--- baseline (doesn't change)
| | new |
| |glyph|
+----+-----+ <--- new descent

A composition rule may have the form (GLOBAL-REF-POINT
NEW-REF-POINT XOFF YOFF), where XOFF and YOFF specify how much
to shift NEW-REF-POINT from GLOBAL-REF-POINT. In this case, XOFF and YOFF are integers in the range -100..100 representing the shifting percentage against the font size.