[Feature] Native Fisheye support

[iandmil]

As far as I can tell, while fisheye images are supported, they are undistorted and rectified before anything else. For best performance, it would be ideal to directly compute features/matches/etc. in distorted image space (a la https://github.com/lsyads/fisheye-ORB-SLAM). Is this currently supported, and if not how challenging would it be to implement?

[matlabbe]

Would need to implement a stereo disparity approach working with not-rectified images. Current implemented approaches try to find the corresponding pixel of a feature in the right image assuming it should be on same line.

Not sure how stereo disparity could be computed in not rectified image, we may need to extract the features from both left and right image, then follow epipolar lines to find the same feature in other image. Or extract features in both image, rectify only features, then compare features on same line in the stereo images.