Screen legendDonald Sutherland battled deadly breathing difficulties for years and was even forced to carry an oxygen tank and tube to help him, RadarOnline.com has learned. Sutherland, who it was announced Thursday had died aged 88, was infamously a four-pack-a-day smoker. Sources told RadarOnline.com the veteran Canadian actor was suffering from an ongoing and obstructive lung condition, chronic obstructive pulmonary disease, or COPD. “It was a debilitating and incurable lung illness, most commonly caused due to smoking,” the insider said. Sutherland, whose career spanned more than six decad…