swscale: Take the destination range into account for yuv->rgb->yuv conversions

The range parameters need to be set up before calling
sws_init_context (which selects which fastpaths can be used;
this gets called by sws_getContext); solely passing them via
sws_setColorspaceDetails isn't enough.

This fixes producing full range YUV range output when doing
YUV->YUV conversions between different YUV color spaces.

Signed-off-by: Martin Storsjö <martin@martin.st>
