please add support for Heretic/Hexen raw screen graphics

[fabiangreffrath]

Hi,

in Heretic and Hexen, full-screen grahpics like e.g. "TITLEPIC" are stored in a "raw" format instead of the usual patch format. That is, the lumps consist of 320x200 bytes, each holding a palette index.

Please allow to access them using the wad.graphics[] API.