Ensure glActiveTexture is cleaned up correctly on functor resume

Change-Id: I103d7d63b17289d599c2c08dcc442cfba9b8e51d
diff --git a/libs/hwui/Caches.h b/libs/hwui/Caches.h
index b7a97ad..f8b1e17 100644
--- a/libs/hwui/Caches.h
+++ b/libs/hwui/Caches.h
@@ -226,6 +226,11 @@
     void activeTexture(GLuint textureUnit);
 
     /**
+     * Invalidate the cached value of the active texture unit.
+     */
+    void resetActiveTexture();
+
+    /**
      * Binds the specified texture as a GL_TEXTURE_2D texture.
      * All texture bindings must be performed with this method or
      * bindTexture(GLenum, GLuint).