graphics/dilay: fix build with GCC-based architectures

GCC needs -Wno-narrowing:
src/isosurface-extraction/grid.cpp:309:53: error: narrowing conversion of '-1' from 'int' to 'char' inside { } [-Wnarrowing]

While here also add USES=gl.

PR: 239312
Approved by: yuri (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D21143