st/mesa: adjust Z coordinates for quad clearing

st/mesa: adjust Z coordinates for quad clearing

Specify the quad's Z position in clip coordinate space, not
normalized Z space.  Use viewport scale, translation = 0.5, 0.5.

Before, we were specifying the quad's Z position in [0,1] and using
viewport scale=1.0, translate=0.0.  That works fine, unless your
driver needs to work in clip coordinate space and needs to
reconstruct viewport near/far values from the scale/translation
factors.  The VMware svga driver falls into that category.

When we did that reconstruction we wound up with near=-1 and far=1
which are outside the limits of [0,1].  In some cases, this caused
the quad to be drawn at the wrong depth.  In other cases it was
clipped away.

Fixes some scissored depth clears with VMware driver.  This should
have no effect on other drivers.  We're already using these values
for the glBitmap and glDraw/CopyPixels code.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>