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.
The test case images are currently not including the text as the example
test run does not include experimental features. This will need to be
updated once the text() feature is not experimental anymore.