[subset] Check for the Null table and not nullptr to detect failure to find a table in glyf accelerator.

Bad bad bad bad code. Don't do that. If compiler's not happy, understand why.

This reverts commit b9dcbb1f.

This is gross. Should be only done if a specific macro is defined. Not the other way
around. This fails my build as autotools companions where not added anyway.

Add a simplistic round() implementation for our purposes, used when the
compiler does not support round() directly.

So I can make a release.

BASE table API will go into hb-ot-layout.h.

Was totally b0rked. Ouch!

Fixes https://github.com/harfbuzz/harfbuzz/issues/863

In preparation to fix https://github.com/harfbuzz/harfbuzz/issues/863

Binary operations are sets are completely broken. Ouch.