Fontbakery: Add check: If hinted, check fonts have head table flag bit 3 set

[davelab6]

Observed behaviour

Per https://docs.microsoft.com/en-us/typography/opentype/spec/head, bit 3 of Head::flags decides whether PPEM should be rounded. This bit should always be set for hinted fonts. We do not have a check for this.

Expected behaviour

We have a check for this.

[felipesanches]

@davelab6, @m4rc1e, @laerm0:
This is the current state of the OFL licenced portion of the Google Fonts collection regarding this new check:

Notes:

• SKIP means the font is not hinted. Detection is based on the presence of a "fpgm" (Font Program) table.
• PASS means the font is hinted and has bit 3 of the head table flags properly set.
• FAIL means the font is hinted but does not set the bit.

[felipesanches]

@davelab6 would you prefer if I opened an issue on google/fonts issue tracker with this kind of collection-wide overview whenever a new check is implemented in fontbakery?

[laerm0]

I'm glad to know our collection is so colorful.

[thundernixon]

I've just come across this while rebuilding Encode with the latest FB. Does anyone know if this can be set in GlyphsApp, or if there's an existing GFtools script to set this?