Enable the fast paths when converting to Rgb565
Enable the fast paths when converting to Rgb565

The generic conversion is too slow for conversions to
RGB565, which are common on embedded platforms (e.g.: Maemo).

This patch enable the fast path for all conversion to rgb_565,
it is a follow-up of 7d7a85fa16b28fdba257bb466be5a6d2b4bf5d2f

Reviewed-by: Tom Cooksey

igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk