[varLib] Optimize gvar isComposite workaround hack

In 9c3dde3 we made varLib write gvar variation entries for
composite glyphs even if they are all-zero vectors.

This is overkill. The requirement from the bug it's trying
to fix is that composite glyphs have some entry in gvar table.
So we just need to write one variation entry per composite
glyph, not for all masters. For fonts with many masters the
empty entries add up.

Fixes https://github.com/fonttools/fonttools/issues/2831