In some cases, calibration gives central point cx,cy != (0.5,0.5), or it
can be decided to crop the input images.
In those cases, it is necessary to split fovx to fovXleft,fovXright and fovy to fovYtop,fovYbottom
Note that the export of cameras to cameras.json merges those values back
to the basic fovx,fovy. This aims at avoiding the modification of
diff_gaussian_rasterization branch used for SIBR_gaussianViewer_app.
Signed-off-by: Matthieu Gendrin <matthieu.gendrin@orange.com>