options: allow use of the max-brightness option for the egl backend

and notify a user better if it's not available

src/options.c

@@ -812,15 +812,14 @@ bool get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
 
 	opt->max_brightness = normalize_d(opt->max_brightness);
 	if (opt->max_brightness < 1.0) {
-		if (opt->use_damage) {
-			log_warn("--max-brightness requires --no-use-damage. Falling "
-			         "back to 1.0");
+		if (opt->backend == BKEND_XRENDER || opt->legacy_backends) {
+			log_warn("--max-brightness is not supported by the %s backend. "
+			         "Falling back to 1.0.",
+			         opt->backend == BKEND_XRENDER ? "xrender" : "legacy glx");
 			opt->max_brightness = 1.0;
-		}
-
-		if (opt->legacy_backends || opt->backend != BKEND_GLX) {
-			log_warn("--max-brightness requires the new glx "
-			         "backend. Falling back to 1.0");
+		} else if (opt->use_damage) {
+			log_warn("--max-brightness requires --no-use-damage. Falling "
+			         "back to 1.0.");
 			opt->max_brightness = 1.0;
 		}
 	}