st/mesa: fix frontbuffer glReadPixels regressions

st/mesa: fix frontbuffer glReadPixels regressions

The change "mesa/readpix: Don't clip in _mesa_readpixels()" caused a
few piglit regressions.  The failing tests use glReadPixels to read
from the front color buffer.  The problem is we were trying to read
from a non-existant front color buffer.  The front color buffer is
created on demand in st/mesa.  Since the missing buffer bounds were
effectively 0 x 0 the glReadPixels was totally clipped and returned
early.

The fix involves creating the real front color buffer when we're about
to try reading from it.

Tested with llvmpipe and VMware driver on Linux, Windows.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94253
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94254
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94257
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Roland Scheidegger <sroland@vmware.com>