EglRenderer: Clear texture before drawing a new frame.

This is necessary in case the drawer doesn't cover all the pixels.

BUG=None

Review-Url: https://codereview.webrtc.org/2704663002
Cr-Commit-Position: refs/heads/master@{#16671}
diff --git a/webrtc/sdk/android/api/org/webrtc/EglRenderer.java b/webrtc/sdk/android/api/org/webrtc/EglRenderer.java
index f05f6bc..f5c1fe1 100644
--- a/webrtc/sdk/android/api/org/webrtc/EglRenderer.java
+++ b/webrtc/sdk/android/api/org/webrtc/EglRenderer.java
@@ -641,6 +641,8 @@
       GLES20.glFramebufferTexture2D(GLES20.GL_FRAMEBUFFER, GLES20.GL_COLOR_ATTACHMENT0,
           GLES20.GL_TEXTURE_2D, bitmapTextureFramebuffer.getTextureId(), 0);
 
+      GLES20.glClearColor(0 /* red */, 0 /* green */, 0 /* blue */, 0 /* alpha */);
+      GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);
       if (frame.yuvFrame) {
         listenerAndParams.drawer.drawYuv(yuvTextures, bitmapMatrix, frame.rotatedWidth(),
             frame.rotatedHeight(), 0 /* viewportX */, 0 /* viewportY */, scaledWidth, scaledHeight);