avutil/utils: run ff_check_pixfmt_descriptors() only when ASSERT_LEVEL is > 0

this safes about 0.1ms

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/utils: fill xyz tables only when they will be used

makes the first call to sws_getContext() 1ms faster

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