vcl: Unbind framebuffer after setting offscreen mode

Change-Id: I057b148f51c8f011cb013e1f06288aec4d9bdb2a

vcl/opengl/gdiimpl.cxx

@@ -292,6 +292,7 @@ void OpenGLSalGraphicsImpl::SetOffscreen( bool bOffscreen )
         glBindFramebuffer( GL_FRAMEBUFFER, mnFramebufferId );
         maOffscreenTex = OpenGLTexture( GetWidth(), GetHeight() );
         glFramebufferTexture2D( GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, maOffscreenTex.Id(), 0 );
+        glBindFramebuffer( GL_FRAMEBUFFER, 0 );
     }
 
     CHECK_GL_ERROR();