Change meaning of cpi->sf.first_step and rename.
Renamed cpi->sf.first_step to cpi->sf.reduce_first_step_size and changed its meaning such that it is a delta applied to reduce the default first step size (>> x) in the motion search rather than an absolute value. The default first step size is already changed according to the image dimensions (smaller for smaller images). cpi->sf.reduce_first_step_size now applies a further correction from the default. Change-Id: Ia94e08bc24c67b604831f980909af7e982fcd16d