Add check for MVAR table with correct values

[davelab6]

Observed behaviour

Per the spec, the MVAR tables contain variation data for metadata otherwise in like OS/2 and hhea; if not present, then the default values in those tables will apply to all instances, which can effect text layout.

Expected behaviour

We must check MVAR tables are present and correct, since text layout software depends on these values

Resources and exact process needed to replicate

https://docs.microsoft.com/en-us/typography/opentype/spec/mvar

[felipesanches]

Similar to #2119, I will call it com.google.fonts/check/varfont/has_MVAR and add it to the Google Fonts specification (a.k.a. "gfonts profile").

[felipesanches]

"Per the spec, the MVAR tables contain variation data for metadata otherwise in like OS/2 and hhea; if not present, then the default values in those tables will apply to all instances, which can effect text layout."

This description is not very clear to be used as a rationale for the check. Do you have a clearer way to express this idea? I will create the check using this text, but I would like to later further improve it.

[felipesanches]

@m4rc1e disagrees with this new check. He said at #2122 (comment):

I disagree with this check.

If we look at the attribs which will trigger MVAR table generation, if they differ, https://github.com/fonttools/fonttools/blob/master/Lib/fontTools/varLib/mvar.py, these should be consistent across a family (according to GF spec).

What do you think, @davelab6 ?