mesa/vbo: Fix scaling issue in 2-bit signed normalized packing.

mesa/vbo: Fix scaling issue in 2-bit signed normalized packing.

Since a signed 2-bit integer can only represent -1, 0, or 1, it is
tempting to simply to convert it directly to a float.  This maps it
onto the correct range of [-1.0, 1.0].  However, it gives different
values compared to the usual equation:

(2.0 *  1.0 + 1.0) * (1.0 / 3.0) = +1.0           (same)
(2.0 *  0.0 + 1.0) * (1.0 / 3.0) = +0.33333333... (different)
(2.0 * -1.0 + 1.0) * (1.0 / 3.0) = -0.33333333... (different)

According to the GL_ARB_vertex_type_2_10_10_10_rev extension, signed
normalization is performed using equation 2.2 from the GL 3.2
specification, which is:

   f = (2c + 1)/(2^b - 1).                                (2.2)

Comments below that equation state: "In general, this representation is
used for signed normalized fixed-point parameters in GL commands, such
as vertex attribute values."  Which is what we're doing here.

The 3.2 specification goes on to declare an alternate formula:

   f = max{c/(2^(b-1) - 1), -1.0}                         (2.3)

which is closer to the existing code, and maps the end points to exactly
-1.0 and 1.0.  Comments below the equation state: "In general, this
representation is used for signed normalized fixed-point texture or
framebuffer values."  Which is *not* what we're doing here.

It then states: "Everywhere that signed normalized fixed-point
values are converted, the equation used is specified."  This is the real
clincher: the extension explicitly specifies that we must use equation
2.2, not 2.3.  So we need to do (2x + 1) / 3.

This matches the behavior expected by oglconform's packed-vertex test,
and is correct for desktop GL (pre-4.2).  It's not correct for ES 3.0,
but a future patch will correct that.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Marek Olšák <maraeo@gmail.com>