graphics/imv: allow building on Wayland-only

imv can work on a system where libX11 is fully removed
and OpenGL is provided by libOpenGL instead of libGL[1].

USE_GL=gl implies lib-depends on libGL, which is built
only when X11 symbols are available, which is not always
the case.

libglvnd provides libOpenGL which provides OpenGL
symbols without X11 symbols.

To satisfy poudriere testport, do USE_GL=egl because
there is no libOpenGL handling in Mk/Uses/gl.mk currently.

libEGL is supposed to always exist per pkg-plist of mesa-*.

Adjust USE_GL for imv to build on global X11=off XCB=off XLIB=off.

[1]: https://github.com/eXeC64/imv/commit/
70a70727b74c532ffb303023b3eb215310948b31

Rebased version is always available here:
https://codeberg.org/ei/ports/commits/branch/ei

PR: 256419
Approved by: dbaio, garga (mentors)
Differential Revision: https://reviews.freebsd.org/D30694