Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11245

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11245

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11245

Also, implement backwards kerning for Format1 in kern and kerx.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1350

We like check_struct() more.

It wasn't doing anything.

Untested.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1255
Fixes https://github.com/harfbuzz/harfbuzz/issues/1252

Fixes https://github.com/harfbuzz/harfbuzz/issues/1247

Disable kern Format2.

Fix kerx Format2.  Manually tested this with Tamil MN font and it works:

$ HB_OPTIONS=aat ./hb-shape Tamil\ MN.ttc -u 0B94,0B95
[tgv_au=0+3435|tgc_ka=1@-75,0+1517]

 HB_OPTIONS=aat ./hb-shape Tamil\ MN.ttc -u 0B94,0B95 --features=-kern
[tgv_au=0+3510|tgc_ka=1+1592]

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.