[software] fix split dual fisheye

[mugulmd]

Description

The dualfisheye split mode of the aliceVision_split360Images software is currently broken, as it only saves on disk two fully black images.
This problem seems to be related to the use of oiio image buffers for copying data.

In this PR, we replace the previous approach with a simpler one: since images are eigen matrices, we copy the blocks we need from the source image and save them, hence there is no need to use oiio at any point.

Notes

Some extra modifications have been made to the aliceVision_split360Images software:

• the extracted images are saved on disk using a rig structure
• the "demo" mode has been renamed to "preview" mode
• the user can now specify the output image extension

These changes of the command-line and output folder structure have 2 side-effects:

• software version went from 1.0 to 2.0
• changes must be reflected in the corresponding Meshroom node.

Meshroom PR: alicevision/Meshroom#1935

[servantftechnicolor]

Although it was not introduced by this PR, i don't understand why the input and outputs are folders instead of sfmData. This make this application difficult to pipeline with the other apps.

[fabiencastan]

Yes, it would be great to also generate an sfmData as suggested by @servantftechnicolor in another PR.

[fabiencastan]

The same rig folder structure should also be used in the case of equirectangular split (not only dual-fisheye).

[fabiencastan]

For another PR: when the input is a folder, we list all files and it would be good to check that the file extension is a supported image file.