Refactor VideoDecoder C++ initialization

[scotts]

In the C++ layer, in VideoDeocder::initializeDecoder(), we were calling std::vector::resize(), adding 1 each time, and then calling std::vector::back() to get a reference to the last element and fill it out. That is not how vectors are designed to be used. Instead, in the loop, we just create a struct on the stack, fill it with values, and call std::vector::push_back().

[NicolasHug]

Curious if we should keep that comment, or what was the reason to remove it? It might be helpful to keep (especially the "Note" part), particularly so that the reader knows that some frame decoding does happen within avformat_find_stream_info()?

I'm finding this from the FFmpeg docs

Some formats do not have a header or do not store enough information there, so it is recommended that you call the avformat_find_stream_info() function which tries to read and decode a few frames to find missing information.

[scotts]

Good catch. I removed the comment because of the latter part about lazy initialization - we're doing that now. But the other stuff is both relevant and informative. I'll submit a PR which revives the relevant stuff.