Fix font display in stereoscopic modes
CGUIFontTTFGL::LastEnd was previously using the relatively high-level
CGraphicContext::SetScissors function to enforce hardware clipping. However,
the coordinates it passed in already contained the stereoscopic offset, so
the CGraphicContext::SetScissors effectively ended up double-applying the
offset, with the effect that clip rectangles were always off-screen.
Changed to call the low-level SetScissors call instead (using g_Windowing to
select the correct implementation, e.g. CRenderSystemGLES::SetScissors).
This also skips the intersection of the scissors with the screen limits, but
that does not appear to matter in practice.