[kern/kerx] Fix offset base

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]
2 files changed