This also adds BaseFilter.pre to specify if the filter is a pre-filter or a post-filter. It is False, for post-filter, by default, as loadFilter() puts filters in postFilters by default.
maybe add another test in integration_test.py testing that the filters parameter is actually passed through to the preprocessor from all the compile* call sites
Continues #466 and is needed for https://github.com/googlefonts/fontmake/pull/737.
This also adds BaseFilter.pre to specify if the filter is a pre-filter or a post-filter. It is False, for post-filter, by default, as
loadFilter()
puts filters in postFilters by default.