Additional metadata field required for correct line-scope display of NTSC and PAL line sample data

[simoninns]

Right now the metadata has white and black 16-bit sample levels included as:

white_16b_ire INTEGER
black_16b_ire INTEGER

The issue with this is that the black level can be 0 or 7.5 IRE depending on the video format (and other factors) - so an additional metadata field is required:

blanking_16b_ire INTEGER

This will contain the 16-bit sample level of the blanking (i.e. 0 IRE generally)

This will enable down-stream tools to correctly represent the video when plotting line scopes.