Capture:
PROC [operator:
PROC[Context], m: Transformation, r:
REAL]
RETURNS [CharMask];
m is the transformation to device coordinates, with x~s and y~f.
Returns NIL if the operator must be re-executed each time, and nothing can be said about the bounding box and/or width.
Returns uncached variant if the operator cannot be cached for some reason (including tryForMask=FALSE), but its bounding box and width are known.
Otherwise returns cached variant; raster IFF (size of raster rep) <= r*(size of runs rep).
The font and char fields of the result are not filled in.