demos: fix multitex.c VertCoord attribute mapping

demos: fix multitex.c VertCoord attribute mapping

If the multitex.vert shader uses the VertCoord generic vertex attribute
instead of the pre-defined gl_Vertex attribute, we need to make sure that
VertCoord gets bound to generic vertex attribute zero.

That's because we need to call glVertexAttrib2fv(0, xy) after all the other
vertex attributes have been set since setting generic attribute 0 triggers
vertex submission.  Before, we wound up issuing the vertex attributes in
the order 0, 1, 2 which caused the first vertex to be submitted before all
the attributes were set.  Now, the attributes are set in 1, 2, 0 order.