mesa: check for z=0 in _mesa_Vertex3dv()

mesa: check for z=0 in _mesa_Vertex3dv()

It's very rare that a GL app calls glVertex3dv(), but one in particular
calls it lot, always with Z = 0.  Check for that condition and convert
the call into glVertex2f.  This reduces VBO memory used and reduces
the number of times we have to switch between float[2] and float[3]
vertex formats in the svga driver.  This results in a small but
measurable performance improvement.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>