small improvements

* optimized fadeToBlackBy() - don't repaint unchanged pixels
* made MIN_HEAP_SIZE configurable by build_flags
* specific error message when not enough memory for LEDs buffer

wled00/data/index.js

@@ -2008,6 +2008,9 @@ function readState(s,command=false)
 		case 36:
 			errstr = "Low Memory (oappend buffer).";
 		  break;
+		case 37:
+			errstr = "no memory for LEDs buffer.";
+		  break;
 		}
 	  showToast('Error ' + s.error + ": " + errstr, true);
 	}