Added Validity Sensors Inc. VFS7552 driver
This sensor can be found in some Dell XPS laptops from 2017 (XPS 15 9560 and XPS 13 9360). GitHub user hmaarrfk reverse engineered the protocol of this specific sensor about two years ago. Based on his work I recently completed writing the accompanying libfprint driver. The sensor is of type FP_SCAN_TYPE_PRESS. In order to get a usable image out of it, the driver first captures the image of the sensor before the finger is placed on it, and then subtracts this reference image from the final image of the fingerprint ("background removal"). Additionally the images captured by the sensor are quite small (112x112 pixels), which would cause problems with the minutiae detection. To circumvent this, the images are scaled up to 4x the original size (doubling of the pixel count on each side).
I already made a pull request on GitHub, but shortly thereafter I realized that the main development happens here on GitLab.