avcodec: remove the VLC DSP mask (except on Android on ARM)

libavutil will always check for all CPU features that it understands,
even those VLC masked. Thus masking features provides no savings.

libavutil runs more tests than VLC. Depending on the platform and
operating system combination, libavutil is either as conservative or
more conservative than VLC. As such masking features provides no extra
safety.

So overall, GetVlcDspMask() is useless, at least nowadays.

Android ARM NEON seems controversial so it is kept for now.
configure: drop old development-only libavcodec versions for VDPAU
hwdummy: remove development versions support
vdpau_avcodec: remove development versions support
libvlc: fix possible free() of static const char[]

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
macosx: do not enforce AR if video view is hidden
macosx: fix page up / page down hotkey support

close #10716
macosx: reorder string functions and add add some documentation

No functional change.