[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

This reverts commit 80af2812, reversing
changes made to c442672e.