Fixed stencil Test example. Writing to stencil buffer was getting disabled after the first frame.

third_party/gles2_book/Chapter_11/Stencil_Test/Stencil_Test.c

 //
 // Book:      OpenGL(R) ES 2.0 Programming Guide
 // Authors:   Aaftab Munshi, Dan Ginsburg, Dave Shreiner
 // ISBN-10:   0321502795
 // ISBN-13:   9780321502797
 // Publisher: Addison-Wesley Professional
 // URLs:      http://safari.informit.com/9780321563835
 //            http://www.opengles-book.com
 //
 
@@ skipping 207 matching lines @@
    //   can test against them without modifying the values we
    //   generated.
    glStencilMask( 0x0 );
    offset += 6;
    for ( i = 0; i < NumTests; ++i )
    {
       glStencilFunc( GL_EQUAL, stencilValues[i], 0xff );
       glUniform4fv( userData->colorLoc, 1, colors[i] );
       glDrawElements( GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, offset );
    }
+   glStencilMask( 0xff );
 }
 
 ///
 // Cleanup
 //
 void stShutDown ( ESContext *esContext )
 {
    STUserData *userData = esContext->userData;
 
    // Delete program object
    glDeleteProgram ( userData->programObject );
 
    // Delete vertex buffer objects
    glDeleteBuffers ( 2, userData->vboIds );
 }