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

graphics/imv/Makefile

@@ -1,6 +1,7 @@
 PORTNAME=	imv
 DISTVERSIONPREFIX=	v
 DISTVERSION=	4.2.0
+PORTREVISION=	1
 CATEGORIES=	graphics
 
 MAINTAINER=	eduardo@FreeBSD.org
@@ -18,7 +19,7 @@ USES=		compiler:c11 desktop-file-utils gettext-runtime gl gnome \
 		meson pkgconfig
 USE_GITHUB=	yes
 GH_ACCOUNT=	eXeC64
-USE_GL=		gl
+USE_GL=		egl
 USE_GNOME=	cairo glib20 pango
 
 PLIST_FILES=	bin/imv \
@@ -42,7 +43,7 @@ WIN_DESC=	Windowing system
 
 ALL_LIB_DEPENDS+=	libwayland-client.so:graphics/wayland
 ALL_USES=	xorg
-ALL_USE=	gl=egl,glu xorg=x11,xcb
+ALL_USE=	gl=glu xorg=x11,xcb
 ALL_MESON_ON=	-Dwindows=all
 ALL_PLIST_FILES=	bin/imv-wayland \
 			bin/imv-x11
@@ -73,8 +74,6 @@ TIFF_MESON_ENABLED=	libtiff
 
 WAYLAND_LIB_DEPENDS=	libwayland-client.so:graphics/wayland \
 			libwayland-egl.so:graphics/wayland
-
-WAYLAND_USE=		gl=egl
 WAYLAND_MESON_ON=	-Dwindows=wayland
 
 X11_USES=	xorg