Merging screen recordings without resampling
There is a command line tool called ffmpeg, which can be installed with any package manager such as brew or macports.
You can use concat option of ffmpeg like this:
ffmpeg -safe 0 -f concat -i listofvideos.txt -c copy output.mov

Contents of listofvideos.txt:
file 'movie1.mov'
file 'movie2.mov'
file 'movie3.mov'

-c copy makes sure the codec is copied and nothing is resampled.
More info about concat can be found here