osg-ffmpeg-videoplayer: Fix crash on ARM

osg-ffmpeg-videoplayer handled frame allocation incorrectly.

It used a vector<uint8_t> as its buffer, meaning the addresses did not respect alignment.

Instead, changes it to use AVFrame as buffers, allocated via av_image_alloc.

We also now only allocate the buffer once, instead of on every frame, which should improve the framerate of videos.

Fixes the following crash on startup on ARM:

Invalid address alignment (signal 7)

Fixes #5807 (closed)