I have a video clip of a car driving by from left to right. My goal is to add a decal (mask) to the car door. Right now I am painstakingly moving the mask via X and Y.
A mostly automatic way to do what you want is to use a Plane Track
The workflow is like this:
Load your video clip in the Movie Clip Editor
Create a tracker on your image by clicking while pressing Ctrl and pressing the tracking forward button (or CtrlT)
Track at least four points within your video corresponding with the area you want to paste your "decal" to.
Select all your trackers, go to the Solve tab and open the Plane Track control and click on Create Plane Track
A new shape will be created. (you might need to adjust the shape to match your needs) The deformation of that plane is controlled by the trackers.
On the compositor use a Plane Track Deform Node (found in the Distort section). Connect it to your decal image, select your video source, camera, plane track, and mix it with the movie clip.
The decal follows the movement and deformation on the video!