[arch-general] libx264 changes

[Carsten Mattner]

Since the libx264 package does not include /usr/include
and it's in x264 now, and because x264 depends on ffmpeg,
I cannot use libx264 without also installing ffmpeg.

For various reasons like missing features and wrong version
I cannot use arch's ffmpeg package and have to build my own
package, and this results in a cyclic dependency I cannot
get out of.

(lib)x264 packages have always been cumbersome due to 8bit
vs 10bit and other issues and are uniquely special in arch,
but since there's already so many x264 packages, let's have
a libx264-headers package. Or keep it in libx264 since 8bit
and 10bit cannot be installed in parallel anyway.

Doug, you probably have good reasons for the recent headers
change and see no way to fix it to make my scenario work, but
I'd appreciate if you gave this a thought and shared your
opinion. If you can't fix this, do you mind explaining what
led to the removal of headers from libx264?

I hope I don't have to build libx264 myself as well since my
PKGBUILD would diverge or force me to build libx264 and x264
custom versions.