Offshore wind could become a cornerstone of the world's power supply as steep cost reductions and improved technology unleash the potential of the green energy source, said the International Energy Agency (IEA) on Friday.Renewables replacing fossil fuel is crucial to meet a globally-agreed goal of limiting temperature rise to below 2 degrees Celsius this century and the expansion of offshore wind could avoid 5-7 billion tonnes of CO2 emissions from the power sector globally…