MC-format binaural rendering output levels

As part of the review of !1280 (merged), it was noticed that the output levels of MC-format rendered to binaural are significantly different. This is obviously problematic as bitrate switching will result in significant changes on the output loudness.

We processed some samples using 5.1 and 7.1+4 input to binaural and measured loudness using BS1770 between the input and output. Data from @laitinenmik will be in comments. The general summary seems to be that the BS1770 values seem to between renderers and MC modes. The differences are up to 6-7 dB.

Somewhat related issues are #837 for ISM input and quite old #248 for SBA.