lavu/mem: extend documentation for av_dynarray_add()

In particular, mention the fact that the amortized element addition cost
is constant.
lavu/mem: fix potential int overflow and crash in av_dynarray_add()

Also extend documentation accordingly.
lavfi/unsharp: use verbal form in help message options
x86: sbrdsp: force PIC addressing for Win64

MSVC complains about the 32bits addressing, while mingw/gcc does not.

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