meta: Convert Z value from normalized to object-space in meta code

Convert Z from a normalized value in the range [0, 1] to an
object-space Z coordinate in [-1, +1] so that drawing at the new Z
position with the default/identity ortho projection results in the
original Z value.  Used by the meta-Clear, Draw/CopyPixels and Bitmap
functions where the Z value comes from the clear value or raster
position.

Fixes piglit tests fdo23670-depth_test, quad-invariance and
glsl-orangebook-ch06-bump as well as oglc zbfunc.c.

https://bugs.freedesktop.org/show_bug.cgi?id=23670

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