flv: Improve log messages

Messages should start with a capital letter and possibly end with a "."
if they are statements.
flv: Warn only once

No point in sending the message multiple time.
flv: Do not mangle dts values for negative cts

Some applications really mean to send negative pts.
lavu: add CHK_OFFS as AV_CHECK_OFFSET to check struct member offsets
lavr: define ResampleContext in resample.h

Required for arch optimized resampling.
aarch64: NEON optimized FIR audio resampling

Optimized for the default filter length 16.

30% faster opus silk decoding.
fate: Add fic-in-avi test

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>