clear all pixels when dispose is 3 but previousImage is null (#2521)

* clear all pixels when dispose is 3 but previousImage is null

* Separate conditions and explain why we need to drop the prev image

* release previousImage for re-use

third_party/gif_decoder/src/main/java/com/bumptech/glide/gifdecoder/StandardGifDecoder.java

@@ -418,8 +418,20 @@ public class StandardGifDecoder implements GifDecoder {
     // Final location of blended pixels.
     final int[] dest = mainScratch;
 
-    // clear all pixels when meet first frame
+    // clear all pixels when meet first frame and drop prev image from last loop
     if (previousFrame == null) {
+      if (previousImage != null) {
+        bitmapProvider.release(previousImage);
+      }
+      previousImage = null;
+      Arrays.fill(dest, COLOR_TRANSPARENT_BLACK);
+    }
+
+    // clear all pixels when dispose is 3 but previousImage is null.
+    // When DISPOSAL_PREVIOUS and previousImage didn't be set, new frame should draw on
+    // a empty image
+    if (previousFrame != null && previousFrame.dispose == DISPOSAL_PREVIOUS
+            && previousImage == null) {
       Arrays.fill(dest, COLOR_TRANSPARENT_BLACK);
     }