implement a txt preview fallback for the case that ttf is not support

lib/private/preview/txt.php

@@ -44,13 +44,20 @@ class TXT extends Provider {
 		$fontFile .= '/../../../core';
 		$fontFile .= '/fonts/OpenSans-Regular.ttf';
 
+		$canUseTTF = function_exists('imagettftext');
+
 		foreach($lines as $index => $line) {
 			$index = $index + 1;
 
 			$x = (int) 1;
 			$y = (int) ($index * $lineSize);
 
-			imagettftext($image, $fontSize, 0, $x, $y, $textColor, $fontFile, $line);
+			if ($canUseTTF === true) {
+				imagettftext($image, $fontSize, 0, $x, $y, $textColor, $fontFile, $line);
+			} else {
+				$y -= $fontSize;
+				imagestring($image, 1, $x, $y, $line, $textColor);
+			}
 
 			if(($index * $lineSize) >= $maxY) {
 				break;