mkv/webm cleanup

[CatKasha]

check EBML header first, then do find and limit find range to 64 bytes (i dont think EBML header can be that large)

also this pr fixes webm detection that created with GStreamer Matroska muxer / GStreamer matroskamux: DocType have webm\x00 so DocType size is different, ex. 0x42, 0x82, 0x85, 0x77, 0x65, 0x62, 0x6D, 0x00