Fix problem when drawing to the Android Media encoder surface.

talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java

Index: talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java
diff --git a/talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java b/talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java
index 47886effd7c36965c31daf7d971a7b3063467f8e..6a46e8626ca43031403a0f488acb68bd570fc77e 100644
--- a/talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java
+++ b/talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java
@@ -382,6 +382,9 @@ public class MediaCodecVideoEncoder {
         mediaCodec.setParameters(b);
       }
       eglBase.makeCurrent();
+      // TODO(perkj): glClear() shouldn't be necessary since every pixel is covered anyway,
+      // but it's a workaround for bug webrtc:5147.
+      GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);
       drawer.drawOes(oesTextureId, transformationMatrix);
       // TODO(perkj): Do we have to call EGLExt.eglPresentationTimeANDROID ?
       // If not, remove |presentationTimestampUs|.