gl-renderer: add EGL surfaceless platform support

This allows passing EGL_PLATFORM_SURFACELESS_MESA to
gl_renderer_display_create(). It is not useful on its own, because the
surfaceless platform has no window surfaces.

This feature will be used by the headless backend.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/renderer-gl/gl-renderer.c

@@ -3303,6 +3303,8 @@ platform_to_extension(EGLenum platform)
 		return "wayland";
 	case EGL_PLATFORM_X11_KHR:
 		return "x11";
+	case EGL_PLATFORM_SURFACELESS_MESA:
+		return "surfaceless";
 	default:
 		assert(0 && "bad EGL platform enum");
 	}
@@ -3369,6 +3371,10 @@ gl_renderer_display_create(struct weston_compositor *ec,
 			return -1;
 	}
 
+	/* Surfaceless is unusable without platform_base extension */
+	if (supports == 0 && platform == EGL_PLATFORM_SURFACELESS_MESA)
+		return -1;
+
 	gr = zalloc(sizeof *gr);
 	if (gr == NULL)
 		return -1;