Fixes https://github.com/harfbuzz/harfbuzz/issues/250

"The values in the right class table are stored pre-multiplied by the
number of bytes in a single kerning value, and the values in the left
class table are stored pre-multiplied by the number of bytes in one
row. This eliminates needing to multiply the row and column values
together to determine the location of the kerning value. The array can
be indexed by doing the right- and left-hand class mappings, adding the
class values to the address of the array, and fetching the kerning
value to which the new address points."

Fix a bug in CBDT sanitize, and redundant check in avar.

There might be a better way to do this, but I couldn't find...

Sorry for the noise, downstream custom builders.  Please adjust.

Issue originally reported at http://cppfiles.com/chromium.txt .

This fixes what appears to be a copy - paste issue which causes both
KernSubTableFormat2::rightClassTable and the 'right' argument to
KernSubTableFormat2::get_kerning to go unused.

Such a headache that Windows defines UINT8, ...; Just prefix it.

Untested, but compiles.

If offset is zero, we return Null() object.  Wasn't prepared for that.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4088

Apparently calibri.ttf does this:
https://github.com/fonttools/fonttools/pull/1094#discussion_r148933791

Almost there.

Pushing this out early to catch bot errors since I'm using template tricks
we were not using in HarfBuzz before.