[subset] zero glyf and loca memory before writing so pads are clean. Test just the part of maxp we care about

[subset] sketch out dropping in-glyf instructions. Sometimes yields differnet size glyphs than fonttools, possibly due to padding not being corrected

Was writing source font's offsets. Ouch.

We use calloc() to allocate simple structs and set them to zero.
For bulk memory, use malloc() that doesn't zero.

auto_array has constructor/destructor. Cannot be used in POD object, as the clang
bots all noticed...

Part of fixing https://github.com/behdad/harfbuzz/issues/585