Add a perspective camera model with non-zero skew

Is your feature request related to a problem? Please describe. Satellite imaging systems often use Rational Polynomial Camera (RPC) models, which are nonlinear. Zhang et al. demonstrated that an RPC model can be approximated numerically over a satellite tile by a linear perspective camera model. However, their method introduces a non-zero skew term which is currently unsupported by COLMAP.

This prevents us from representing these approximated cameras accurately in COLMAP and limits its use in remote sensing reconstruction workflows.

Describe the solution you'd like In order to enable COLMAP in remote sensing applications we would like to see the addition of a skew-perspective camera model with $f_x, f_y, c_x, c_y, s$ where $s$ is the non-zero skew term. This camera model could most likely be derived from the existing BasePerspectiveCameraModel.

Describe alternatives you've considered Our current workaround is to vendor and patch COLMAP. This allows us to support the required camera geometry, but maintaining a downstream fork is not a sustainable long-term solution.

Additional context A comprehensive set of patches are maintained by SBCV for this use case in ColmapForVisSatPatches. Those patches may provide a useful implementation reference.

A similar capability also appears to have been requested previously in issue #490.