Add glyph-images of HNG-KHM.

Add glyph-images of HNG-KHM.
(U+76F8-var-001): New glyph node to integrate HNG-KHI0504-0 and
HNG-KHM0455-0; add `<-subsumptive' for U+76F8.
(U+7700): Add `<-formed' for U+660E.
(U+7758): Use `->denotational' and `->subsumptive'.
(HNG-KHM0458-0): New character; add `<-formed' for U+777A.
(U+777A): Use `->subsumptive'.
(U-00025211): Add representative nodes.
(U+77BB-itaiji-001): New character to integrate HNG-KHI0122-0 and
HNG-KHM0097-0; add `<-denotational' for BUCS+77BB; add `<-formed' for
U+77BB.