Add support for VARIANT x FILE TYPE parallel builds

[chrissimpkins]

Currently fontmake-mp supports parallel builds across font variants only. This means that if you request both *.otf and *.ttf builds across four variant types, the compiles are executed in four processes and both *.otf and *.ttf builds occur in a variant-specific process. This change will lead to parallel builds across every font variant x file build type request in a command and should significantly decrease build times for requests that include both *.otf + *.ttf compiles on machines with CPU number > number of font variants in the request.