Fix nalu parsing for 3-byte & 4-byte start seqs

[rileym-arcules]

Description

There is an issue in the current emitNalu function where the implementation can skip over 3-byte start sequences.

The emitNalu function was changed in commit bfe92b9 which introduced this regression: bfe92b9#diff-5eca440e933baa4c5491dcd9ccc7c1f4d807994e6c5b54c304228c34fcc3c963L40

From analysis of the current emitNalu function,

rtp/codecs/h264_packet.go

Lines 45 to 67 in ee5524b

	func emitNalus(nals []byte, emit func([]byte)) {
	start := 0
	length := len(nals)
	for start < length {
	end := bytes.Index(nals[start:], annexbNALUStartCode)
	offset := 4
	if end == -1 {
	end = bytes.Index(nals[start:], naluStartCode)
	offset = 3
	}
	if end == -1 {
	emit(nals[start:])
	break
	}
	emit(nals[start : start+end])
	// next NAL start position
	start += end + offset
	}
	}

one can observe that NALUs with 3-byte start sequences may be skipped over if it is the first NALU, or if it is between two NALUs with 4-byte start sequences (in this case: at L52 end != -1)

This PR aims to resolve this issue by searching for 3-byte start sequences and accounting for 4-byte sequences afterward.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.76%. Comparing base (ee5524b) to head (5662773).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage   86.69%   86.76%   +0.07%     
==========================================
  Files          26       26              
  Lines        3043     3061      +18     
==========================================
+ Hits         2638     2656      +18     
  Misses        348      348              
  Partials       57       57              

Flag	Coverage Δ
go	86.76% <100.00%> (+0.07%)	⬆️
wasm	86.21% <100.00%> (+0.08%)	⬆️

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