swscale/swscale: clear to/from convert pointers when they used.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale: factor ff_sws_init_range_convert() out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/utils: move handling of unscaled case to the end of init function

This way all context fields get initialized and can be used without needing to reinit the context

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/utils: fix changing src/dst range after initializing the context

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>