Preserve transparency when loading from a file

Fix #7148 - again :)

lib/private/image.php

@@ -450,6 +450,9 @@ class OC_Image {
 			case IMAGETYPE_GIF:
 				if (imagetypes() & IMG_GIF) {
 					$this->resource = imagecreatefromgif($imagePath);
+					// Preserve transparency
+					imagealphablending($this->resource, true);
+					imagesavealpha($this->resource, true);
 				} else {
 					OC_Log::write('core',
 						'OC_Image->loadFromFile, GIF images not supported: '.$imagePath,
@@ -468,6 +471,9 @@ class OC_Image {
 			case IMAGETYPE_PNG:
 				if (imagetypes() & IMG_PNG) {
 					$this->resource = imagecreatefrompng($imagePath);
+					// Preserve transparency
+					imagealphablending($this->resource, true);
+					imagesavealpha($this->resource, true);
 				} else {
 					OC_Log::write('core',
 						'OC_Image->loadFromFile, PNG images not supported: '.$imagePath,

tests/lib/image.php

@@ -115,6 +115,9 @@ class Test_Image extends PHPUnit_Framework_TestCase {
 	public function testData() {
 		$img = new \OC_Image(OC::$SERVERROOT.'/tests/data/testimage.png');
 		$raw = imagecreatefromstring(file_get_contents(OC::$SERVERROOT.'/tests/data/testimage.png'));
+		// Preserve transparency
+		imagealphablending($raw, true);
+		imagesavealpha($raw, true);
 		ob_start();
 		imagepng($raw);
 		$expected = ob_get_clean();