ffmpeg: Improve Cgo logging

[jailuthra]

Why
Clean up logging in the Cgo code (Fixes #209)

What

• Use ffmpeg's logging library with appropriate levels like ERROR, WARNING, INFO instead of throwing everything on stderr blindly.
• Add a well defined log prefix like [lpms.decoder.xyz] of the component from where the log is being emitted.
• Does not add any extra INFO logs, will do it separately the next time I go around debugging something and need to trace out the location in the transcoder pipeline

Notes
Commit(s) to be applied on top of #212

[oscar-davids]

I think it's a must for code maintenance and debugging.

As we know, ffmpeg logging is leveled by the "av_log_set_level" function. But this function has been hard-coded as "AV_LOG_WARNING" in our C level lpms_init() function.
I think it would be better to redefine the C level lpms_init() function and the go level InitFFmpeg() function for outputting the log.

type LogLevel int

const (
LogPanic LogLevel = iota
LogFatal
LogError
LogWarning
LogInfo
LogVerbose
LogDebug
LogTrace
)

func InitFFmpeg(level LogLevel ) {
...
}

Along with this, I think that if some stuff discussed in #212 would be supplemented, it would be a better development environment.

[jailuthra]

But this function has been hard-coded as "AV_LOG_WARNING" in our C level lpms_init() function.

@oscar-davids Ah yes I had thought about making that match the golang's glog logging level but left it as-is because matching the levels might lead to really verbose ffmpeg logs.

But the suggestion you make around making it separately configurable using a golang enum sounds like a good way to go! Will do this in the update commit.

I think it would be better to redefine the C level lpms_init() function and the go level InitFFmpeg() function for outputting the log.

[jailuthra]

Updates:

• Create shared #define macros for the logging functions for ERROR, WARNING and INFO levels
• Make logging level configurable via the C and Golang module's init() methods