This adds a simple detection logic based on the hb_unicode_script()
function provided by harfbuzz. Automatic script selection will only
happen if no valid parameter was given (basically only in case of
a missing script parameter in text()).
If different scripts are found in a single text, the resulting value
is HB_SCRIPT_UNKNOWN.
Some fonts with symbol encoding still have the charcodes in the ASCII
area instead of Unicode page 0xf000. To make those work too, the special
handling is only enabled in case the first charcode is >= 0xf000.