swscale: Replace illegal vector keyword usage in altivec code

swscale: Replace illegal vector keyword usage in altivec code

While this technically compiles in current ffmpeg, this is only
because ffmpeg is compiled in strict ISO C mode, which disables
the builtin 'vector' keyword for AltiVec/VSX. Instead this gets
replaced with a macro inside altivec.h, which defines vector to
be actually __vector, which accepts random types.

Normally, the vector keyword should be used only with plain
scalar non-typedef types, such as unsigned int. But we have the
vec_(s|u)(8|16|32) macros, which can be used in a portable manner,
in util_altivec.h in libavutil.

This is also consistent with other AltiVec/VSX code elsewhere in
the tree.

Fixes #7861.

Signed-off-by: Daniel Kolesa <daniel@octaforge.org>
Signed-off-by: Lauri Kasanen <cand@gmx.com>