add windows and mac builds to ci (#873)

* add windows and mac builds to ci
* fall back to mac 10.15
* split lcov target out from the ci-postbuild target
* Specify which build does C++ coverage w/ lcov
* move the env config out to the top level
* sanitize paths when writing autogen docs
* windows is stricter about using file handles that are already open, so
  switching to a pattern where TemporaryDirectory is used to create
  things should be safer.  Its also cleaner than the mkdtemp/try/except
  pattern that had been used in the tests before.
* remove BOM from example xml file
* only run XGES unit tests on linux w/ python3

Author: ssteinbach

tests/sample_data/premiere_generators.xml

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="UTF-8"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE xmeml>
 <xmeml version="4">
 	<sequence id="sequence-1" TL.SQAudioVisibleBase="0" TL.SQVideoVisibleBase="0" TL.SQVisibleBaseTime="0" TL.SQAVDividerPosition="0.5" TL.SQHideShyTracks="0" TL.SQHeaderWidth="236" Monitor.ProgramZoomOut="9068963904000" Monitor.ProgramZoomIn="0" TL.SQTimePerPixel="0.071968375881910282" MZ.EditLine="8793504720000" MZ.Sequence.PreviewFrameSizeHeight="1080" MZ.Sequence.PreviewFrameSizeWidth="1920" MZ.Sequence.AudioTimeDisplayFormat="200" MZ.Sequence.PreviewRenderingClassID="1297106761" MZ.Sequence.PreviewRenderingPresetCodec="1297107278" MZ.Sequence.PreviewRenderingPresetPath="EncoderPresets/SequencePreview/cc7991f5-c236-4db1-957e-2c71f924e81c/I-Frame Only MPEG.epr" MZ.Sequence.PreviewUseMaxRenderQuality="false" MZ.Sequence.PreviewUseMaxBitDepth="false" MZ.Sequence.EditingModeGUID="cc7991f5-c236-4db1-957e-2c71f924e81c" MZ.Sequence.VideoTimeDisplayFormat="110" MZ.WorkOutPoint="15235011792000" MZ.WorkInPoint="0" MZ.ZeroPoint="0" explodedTracks="true">