Export speed-up and render dialog improvements

[firthm01]

Export was very slow with the binaural plugin.

83 has been open for a while which advised disabling monitoring plugins to squeeze more export performance and this is even more relevant now with the heavy load of the bin mon plugin. It also advised on ways to make the waveform display during rendering more relevant. This PR fixes both issues in a slightly different way:

Monitoring bypass

• Scene knows when it is exporting due to connection to extension. It passes this state in monitoring metadata.
  • It does not bother sending any more metadata until exporting is finished (no point - mon plugins don't need it)
• Monitoring backends (both LS and Bin) read and store this state.
• Monitoring AudioProcessors refer to that state to determine what to do during processBlocks:
  • If not exporting, method runs as normal
  • If exporting, input channels are simply summed and copied to all output channels. This is fast and bypasses BEAR/EAR signal processing, but still produces a relevant output (see below).

Render dialog

• Stereo output waveform was confusing (why stereo, when this is ADM?). Force mono in render dialog so that we only see one waveform during rendering.
  • The "Auto" text we used before was obviously invalid and this made REAPER default to Stereo. We set mono and hide the channels combobox and label (otherwise that's also confusing).
• With channels simply summed by mon plugins during export, the waveform shown in the render dialog is somewhat representative of content.

Other fixes:

• Correct resetting of state when "ADM BW64" option is selected in dialog, then unselected, then selected again.

Closes #83

[firthm01]

Just a note on this - might be worth actually testing on MacOS 12+ in case any of the swell stuff behaves differently (tested fine on MacOS 10.13). When selecting "ADM BW64" in the render dialog, should see;

• Selecting "Source" as "Master mix" and disabling combobox
• Selecting "Bounds" as "Entire project" and disabling combobox
• "Presets" button disabled
• Selecting the current project/device sample rate as the export "Sample rate:" and disabling combobox
• Selecting "Mono" as "Channels:" and HIDING both combobox and label (you'll see a single waveform during the actual render to confirm mono was successfully selected)
• Undoing all of the above when selecting another format