Define encode-composition-rule and find-composition

Define encode-composition-rule and find-composition
for Emacs 20.4 and the earlier versions.
(ps-mule-init-external-library): Just require a feature for
external libraries.
(ps-mule-prologue): Postscript code modified for new composition.
(ps-mule-find-wrappoint): New arg COMPOSITION.
(ps-mule-plot-string): Delete code for composite characaters.
(ps-mule-plot-composition): New funcion.
(ps-mule-prepare-font-for-components): New function.
(ps-mule-plot-components): New function.
(ps-mule-composition-prologue-generated): Renamed from
ps-mule-cmpchar-prologue-generated.
(ps-mule-composition-prologue): New named from
ps-mule-cmpchar-prologue.  Modified for new composition.
(ps-mule-plot-rule-cmpchar, ps-mule-plot-cmpchar,
ps-mule-prepare-cmpchar-font): Deleted.
(ps-mule-string-encoding): New arg NO-SETFONT.
(ps-mule-bitmap-prologue): In Postscript code of BuildGlyphCommon,
check Composing, not Cmpchar
(ps-mule-initialize): Set ps-mule-composition-prologue-generated
to nil.
(ps-mule-begin-job): Check existence of new composition.