FS-7338 #comment disable mod_bv by default since the lib is not available on all supported platforms

FS-7500: switch_img_overlay now support ARGB and negtive position, also change alpha value to match doc description

need -lswscale to converto from non-I420 fmt to I420

FS-7509: add userVariables parser to initial connection all variables set in this obj will be set on every inbound call