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>

src/mesa/main/api_loopback.c

@@ -629,7 +629,10 @@ _mesa_Vertex2sv( const GLshort *v )
 void GLAPIENTRY
 _mesa_Vertex3dv( const GLdouble *v )
 {
-   VERTEX3( (GLfloat) v[0], (GLfloat) v[1], (GLfloat) v[2] );
+   if (v[2] == 0.0)
+      VERTEX2( (GLfloat) v[0], (GLfloat) v[1] );
+   else
+      VERTEX3( (GLfloat) v[0], (GLfloat) v[1], (GLfloat) v[2] );
 }
 
 void GLAPIENTRY