Fix the texture ID reuse issue in HWUI.

Issue: When the layer of previous frame is destroyed, it doesn't clear the
texture id in mBoundTextures[mTextureUnit], so in the next frame, if
glGenTexture returns same texture ID of the previous frame,
the new texture is not bound.

CRs-fixed: 671736

Change-Id: Ifc5fd2115fb1863b3c006ab14b0faabeaeb4eab4
diff --git a/libs/hwui/Caches.h b/libs/hwui/Caches.h
index 282aee9..544757a 100644
--- a/libs/hwui/Caches.h
+++ b/libs/hwui/Caches.h
@@ -264,6 +264,11 @@
     void resetBoundTextures();
 
     /**
+     * Clear the cache of bound textures.
+     */
+    void unbindTexture(GLuint texture);
+
+    /**
      * Sets the scissor for the current surface.
      */
     bool setScissor(GLint x, GLint y, GLint width, GLint height);