fixed roll operator bugs uncovered by the test

Ensure that we have the variables at the beginning of the block.  These
are the only fixes that we need for building HarfBuzz on older
compilers.

Instead of passing a CFLAG/CXXFLAG to define HB_EXTERN, define it
directly in src/hb.hh as __declspec(dllexport) extern when we are
building HarfBuzz as DLLs on Visual Studio.  Define HB_INTERNAL
as nothing without defining HB_NO_VISIBILITY when building HarfBuzz as
DLLs to avoid linker errors on Visual Studio builds.

Also "install" harfbuzz-subset.dll into $(PREFIX)\bin as the
hb-subset utility will depend on that DLL at runtime, when HarfBuzz is
built as DLLs.  Since it consists of private APIs that are subject to
change, we do not install its headers nor .lib file.

Ouch!

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

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

added source hb-ot-cff2-table.cc
augmented VariationData to return scalars
misc bug fixes, renaming, cleanup

"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."