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

Fixes https://github.com/harfbuzz/harfbuzz/issues/1232

"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."

Deleted recently added API:
    hb_buffer_set_invisible_codepoint()
    hb_buffer_get_invisible_codepoint()

    hb-shape / hb-view --invisible-codepoint

New API:
    hb_buffer_set_invisible_glyph()
    hb_buffer_get_invisible_glyph()

    hb-shape / hb-view --invisible-glyph

Fixes https://github.com/harfbuzz/harfbuzz/issues/1216

This reverts commit f62f6e90.

"Improve" is a strong word in this case though, I understand.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1224

We use scratch-flags to short-circuit this function.  No need for previous
early loop.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1216

New API:
hb_buffer_set_invisible_codepoint()
hb_buffer_get_invisible_codepoint()

hb-shape / hb-view --invisible-codepoint

Since we have get_*_advanes() API now, the overhead is once per shape,
not once per glyph.

Only cmap is warmed-up at set_funcs() time now.