Fix FBO restoring in QOpenGLTextureGlyphCache

QOpenGLTextureGlyphCache::restoreTextureData restores FBO which
was binded before restoreTextureData call. More specifically,
it restores QOpenGLContextPrivate's current_fbo member. This works
if FBO was binded by QOpenGLFramebufferObject but not if FBO was
binded using glBindFramebufferObject and rendering done via
QOpenGLPaintDevice.

This patch fixes it by querying current FBO using
GL_FRAMEBUFFER_BINDING query and restoring it.

Change-Id: Ia97a21e62566dc39a5191b66d3ca0e3ad0845ce1
Reviewed-by: Valery Volgutov <valery.volgutov@lge.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>