This fixes the following compiler error, if compiling with MSVC
for ARM (32 bit):
src/libswscale/ops_chain.c(48): error C2719: 'priv': formal parameter with requested alignment of 16 won't be aligned
This change shouldn't affect the performance of this operation
(which in itself probably isn't relevant); instead of copying the
contents of the SwsOpPriv struct from the stack as parameter,
it gets copied straight from the caller function's stack frame
instead.
Separately from this issue, MSVC 17.8 and 17.9 end up in an
internal compiler error when compiling libswscale/ops.c, but
older and newer versions do compile it successfully.
