Support album artist in WavPack tags
Use memset() to zero out the WavPack read buffer
Clear empty tag fields when saving WavPack tags

Refactor WavPack tag saving to use a new
et_wavpack_append_or_delete_tag_item() helper function. Call
WavpackDeleteTagItem() to delete a tag item if the item is empty.
Read the WavPack channel mask

Use WavpackGetChannelMask() to read the channel mask from the WavPack
header, and add a new et_wavpack_channel_mask_to_string() function to
format it for display.
Fix string leak when reading WavPack tags