math/saga: Update to 7.8.0

Minor release with several bug fixes and some enhancements like[1]:

• Added OpenCV version 4 support
• Separate grid system for weather data in sim_hydrology/overland_flow
• Reworked, free positioning for layout items (map, legend, titel, ...) in print layout of saga_gui
• Added SAGA4QGIS, updated for inno-setup v6 in accessories/helper

[1] https://sourceforge.net/p/saga-gis/wiki/Changelog%207.8.0/attachment/changelog_saga_7.8.0.txt

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D26737