vtdec: add support for level 6 6.1 and 6.2

Add level 6, 6.1 and 6.2 levels in get_dpb_max_mb_s_from_level to fix decoding with vtdec.

GST_DEBUG=3 /Library/Frameworks/GStreamer.framework/Commands/gst-launch-1.0 filesrc location=/Users/andoni/Downloads/rewoo_full-2.mp4 ! qtdemux ! vtdec ! glimagesink
Setting pipeline to PAUSED ...
0:00:00.102259000 28394    0x13780b1e0 WARN                 basesrc gstbasesrc.c:3695:gst_base_src_start_complete:<filesrc0> pad not activated yet
Pipeline is PREROLLING ...
Got context from element 'sink': gst.gl.GLDisplay=context, gst.gl.GLDisplay=(GstGLDisplay)"\(GstGLDisplayCocoa\)\ gldisplaycocoa0";
0:00:00.106533000 28394    0x138836cc0 WARN                 qtdemux qtdemux_types.c:267:qtdemux_type_get: unknown QuickTime node type sgpd
0:00:00.106548000 28394    0x138836cc0 WARN                 qtdemux qtdemux_types.c:267:qtdemux_type_get: unknown QuickTime node type sbgp
0:00:00.106778000 28394    0x138836cc0 WARN                 qtdemux qtdemux.c:3249:qtdemux_parse_trex:<qtdemux0> failed to find fragment defaults for stream 1
0:00:00.107901000 28394    0x138836cc0 WARN                 qtdemux qtdemux.c:3249:qtdemux_parse_trex:<qtdemux0> failed to find fragment defaults for stream 2
0:00:00.109474000 28394    0x138836cc0 FIXME           videodecoder gstvideodecoder.c:1199:gst_video_decoder_drain_out:<vtdec0> Sub-class should implement drain()
0:00:00.110724000 28394    0x138836cc0 ERROR                  vtdec vtdec.c:1102:get_dpb_max_mb_s_from_level:<vtdec0> unknown level 60
0:00:00.110732000 28394    0x138836cc0 WARN                   vtdec vtdec.c:1169:get_h264_dpb_size_from_sps:<vtdec0> error: invalid level found in SPS, could not compute max_dpb_mbs
ERROR: from element /GstPipeline:pipeline0/GstVtdec:vtdec0: Could not decode stream.
Additional debug info:
../sys/applemedia/vtdec.c(1169): get_h264_dpb_size_from_sps (): /GstPipeline:pipeline0/GstVtdec:vtdec0:
invalid level found in SPS, could not compute max_dpb_mbs
ERROR: pipeline doesn't want to preroll.
0:00:00.110784000 28394    0x138836cc0 WARN                   vtdec vtdec.c:1237:compute_h264_decode_picture_buffer_length:<vtdec0> Failed to get DPB size from SPS
Setting pipeline to NULL ...
0:00:00.166631000 28394    0x13803b6d0 ERROR        glcaopengllayer gstglcaopengllayer.m:161:-[GstGLCAOpenGLLayer copyCGLContextForPixelFormat:]: failed to retrieve GStreamer GL context in CAOpenGLLayer
0:00:00.226834000 28394    0x138836cc0 WARN            videodecoder gstvideodecoder.c:934:gst_video_decoder_setcaps:<vtdec0> Subclass refused caps
Freeing pipeline ..