Prepare release notes for Libav 10 "Eks"

Additional editing by Diego Biurrun
fate: Explicitly specify the pixel format for the pngparser test

The RGB32 pixel format is RGBA/BGRA depending on target
endianness - make sure to convert it to one specific format for
the framecrc tests.

This fixes the pngparser fate test on big endian.

Signed-off-by: Martin Storsjö <martin@martin.st>