Specialize the fast_composite_scaled_nearest_* scalers to positive x units

Specialize the fast_composite_scaled_nearest_* scalers to positive x units

This avoids a test in the inner loop, which improves performance
especially for tiled sources.

On x86-32, I get these results:

Before:
op=1, src_fmt=20028888, dst_fmt=20028888, speed=306.96 MPix/s (73.18 FPS)
op=1, src_fmt=20028888, dst_fmt=10020565, speed=102.67 MPix/s (24.48 FPS)
op=1, src_fmt=10020565, dst_fmt=10020565, speed=324.85 MPix/s (77.45 FPS)

After:
op=1, src_fmt=20028888, dst_fmt=20028888, speed=332.19 MPix/s (79.20 FPS)
op=1, src_fmt=20028888, dst_fmt=10020565, speed=110.41 MPix/s (26.32 FPS)
op=1, src_fmt=10020565, dst_fmt=10020565, speed=363.28 MPix/s (86.61 FPS)