This step applies only to stencil index groups.
If MAP_STENCIL is enabled,
then the index is looked up in the PIXEL_MAP_S_TO_S
table (otherwise, the index is not looked up).
The table must have 
entries for some integer n,
and the integer part of the index is ANDed with 
,
producing a value.
This value addresses the table,
and the value in the table replaces the index.
The floating-point table value is first rounded to a fixed-point
value with unspecified precision.