Legacy OpenGL preference mismatch between Minetest and IrrlichtMt

[JosiahWI]

Minetest version

544b9d5

IrrlichtMt version

2fec5e5

OS / Hardware

Operating system: N/A
CPU: N/A

GPU model: N/A
OpenGL version: N/A

Summary

Minetest has a cache variable OPENGL_GL_PREFERENCE that determines whether Legacy OpenGL is preferred or not, but IrrlichtMt always prefers Legacy OpenGL regardless of Minetest's cache variable. Since IrrlichtMt always wants the Legacy version anyway, my suggested fix is to remove the cache option and let IrrlichtMt handle the OpenGL requirements entirely. The comments could be preserved by moving them to IrrlichtMt, or by requesting the Legacy version (no effect; for style only) with the appropriate comments before find_package(IrrlichtMt).

[rubenwardy]

That variable was set to silence a CMake warning