st/mesa: use _mesa_RasterPos() when possible

st/mesa: use _mesa_RasterPos() when possible

The st_RasterPos() function goes to great pains to implement the
rasterpos transformation.  It basically uses gallium's draw module to
execute the vertex shader to draw a point, then capture that point's
attributes.

But glRasterPos isn't typically used with a vertex shader so we can
usually use the old/fixed-function implementation which is a lot simpler
and faster.

This can add up for legacy apps that make a lot of calls to glRasterPos.

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