#27 mt_RsrcUserDraw() overwrites previously G_USERDEF data

[Arnaud BERCEGEAY]

when mt_RsrcUserDraw() is invoked on a USERDEF object which was created by windom (box with 3d rendering for example), the userdef data attached to this USERDEF object are lost because user->ub_parm are overwritten without checking... and so, these userdef data are never free-ed (reported by m0n0).
This may also affect USERDEF already declared by RsrcUserDraw().

Maybe a check of user->ub_parm should be added in mt_RsrcUserDraw(), and a note in the doc should be added to remember that the G_USERDEF must be an "empty" USERDEF (with no data attached), and fix RsrcFree to set back ub_parm to 0 after being free()-ed.