Fix lost transparency when converting to GIF format

phpunit.dist.xml

@@ -4,6 +4,9 @@
     <testsuite name="Unit Tests">
       <directory suffix=".php">./tests/Unit</directory>
     </testsuite>
+    <testsuite name="Feature Tests">
+      <directory suffix=".php">./tests/Feature</directory>
+    </testsuite>
   </testsuites>
   <source>
     <include>

src/Drivers/Gd/Encoders/GifEncoder.php

@@ -6,6 +6,7 @@
 
 use Exception;
 use Intervention\Gif\Builder as GifBuilder;
+use Intervention\Image\Drivers\Gd\Cloner;
 use Intervention\Image\EncodedImage;
 use Intervention\Image\Encoders\GifEncoder as GenericGifEncoder;
 use Intervention\Image\Exceptions\EncoderException;
@@ -26,11 +27,10 @@ public function encode(ImageInterface $image): EncodedImage
             return $this->encodeAnimated($image);
         }
 
-        $gd = $image->core()->native();
+        $gd = Cloner::clone($image->core()->native());
         $data = $this->buffered(function () use ($gd) {
             imageinterlace($gd, $this->interlaced);
             imagegif($gd);
-            imageinterlace($gd, false);
         });
 
         return new EncodedImage($data, 'image/gif');

tests/Feature/Gd/ConvertPngGif.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Intervention\Image\Tests\Feature\Gd;
+
+use Intervention\Image\ImageManager;
+use Intervention\Image\Tests\GdTestCase;
+
+class ConvertPngGif extends GdTestCase
+{
+    public function testConversionKeepsTransparency(): void
+    {
+        $converted = ImageManager::gd()
+            ->read(
+                $this->readTestImage('circle.png')->toGif()
+            );
+
+        $this->assertTransparency($converted->pickColor(0, 0));
+        $this->assertColor(4, 2, 4, 255, $converted->pickColor(25, 25), 4);
+    }
+}

tests/Feature/Imagick/ConvertPngGif.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Intervention\Image\Tests\Feature\Imagick;
+
+use Intervention\Image\ImageManager;
+use Intervention\Image\Tests\ImagickTestCase;
+
+class ConvertPngGif extends ImagickTestCase
+{
+    public function testConversionKeepsTransparency(): void
+    {
+        $converted = ImageManager::imagick()
+            ->read(
+                $this->readTestImage('circle.png')->toGif()
+            );
+
+        $this->assertTransparency($converted->pickColor(0, 0));
+        $this->assertColor(4, 2, 4, 255, $converted->pickColor(25, 25), 4);
+    }
+}