Add default EGL/GL library paths for Cocoa-less macOS builds.

Without SDL_VIDEO_DRIVER_COCOA the default-library chain falls through to
the desktop-Linux branch (libEGL.so.1 etc.), which dlopen() can never find
on macOS. Point the defaults at the MacPorts Mesa dylibs; @PREFIX@ is
substituted in post-patch. The SDL_VIDEO_DRIVER_COCOA (ANGLE) branch above
still takes precedence when Cocoa is enabled. Every branch must define
DEFAULT_EGL and all three DEFAULT_OGL_ES* macros - their use sites are not
ifdef-guarded; only DEFAULT_OGL/ALT_OGL are optional. The ES_PVR name is a
second-chance fallback tried after DEFAULT_OGL_ES fails, so it simply
repeats the GLESv1 path here.

--- src/video/SDL_egl.c
+++ src/video/SDL_egl.c
@@ -95,6 +95,14 @@
 #define DEFAULT_OGL_ES_PVR "libGLES_CM.dylib"   //???
 #define DEFAULT_OGL_ES     "libGLESv1_CM.dylib" //???
 
+#elif defined(SDL_PLATFORM_MACOS)
+// Mesa EGL and GL(ES) dylibs from MacPorts (Cocoa-less X11 + Wayland builds)
+#define DEFAULT_OGL        "@PREFIX@/lib/libGL.1.dylib"
+#define DEFAULT_EGL        "@PREFIX@/lib/libEGL.1.dylib"
+#define DEFAULT_OGL_ES2    "@PREFIX@/lib/libGLESv2.2.dylib"
+#define DEFAULT_OGL_ES_PVR "@PREFIX@/lib/libGLESv1_CM.1.dylib"
+#define DEFAULT_OGL_ES     "@PREFIX@/lib/libGLESv1_CM.1.dylib"
+
 #elif defined(SDL_PLATFORM_OPENBSD)
 // OpenBSD
 #define DEFAULT_OGL        "libGL.so"