package/fbgrab: bump version to 1.3.1 and update projct URL

- bump version to 1.3.1
  Changelog:
  * Incorrect alpha value when converting 32-bit framebuffers.
  * Documentation for github instead of own homepage.

- update project URL

Fixes bug 12606 ([1]).

[1] https://bugs.busybox.net/show_bug.cgi?id=12606



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Tested-by: Timo Ketola <timo.ketola@exertus.fi>
Acked-by: Timo Ketola <timo.ketola@exertus.fi>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>