[subset] sketch out support for a call that lists all codepoints present in a font. Implement support for it in format 4 cmap sub table.

* [subset] sanitize individual DeviceRecord's as part of hdmx sanitization.

* [subset] Fix out of bounds read with non-two byte align glyphs.

* [subset] Just use size_device_record >= DeviceRecord::min_size.

* [subset] Add TODO.

* [subset] Re-order checks in hdmx sanitize.

Before 1.7.5, we were setting -fno-exceptions etc on CXXFLAGS. In 1.7.6
we set it as CPPFLAGS. Try fixing. Also, I'm fairly sure it's safe to
set these unconditionally.

Fixes https://github.com/harfbuzz/harfbuzz/issues/880 (or so I hope)

They are handled in our common header.

Include stdbool.h in hb-setset-test.h instead of in the individual
sources, if it is found; otherwise use a simplistic fallback for it if
it is not found.

Also declare variables at the top of the block, to build on pre-C99
compiliers.

For the API tests, output the test programs at $(TOP_BUILDDIR) so that
the freshly-built DLLs will be available for the test programs.  For
those that are run through the Python wrapper scripts, use
${PYTHON_EXECUTABLE} instead of plain 'python' in case the Python
interpreter is not in the PATH.

[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