mesa/vbo: Fix scaling issue in 10-bit signed normalized packing.

mesa/vbo: Fix scaling issue in 10-bit signed normalized packing.

For the 10-bit components, the divisor was incorrect.  A 10-bit signed
integer can represent -2^9 through 2^9 - 1, which leads to the following
ranges:

       (float)value.x          -> [ -512,  511]
2.0F * (float)value.x          -> [-1024, 1022]
2.0F * (float)value.x + 1.0F   -> [-1023, 1023]

So dividing by 511 would incorrectly scale it to approximately:
[-2.001956947, 2.001956947].  To correctly scale to [-1.0, 1.0], we need
to divide by 1023.

This correctly implements the desktop GL rules.  ES 3.0 has different
rules, but those will be implemented in a separate patch.

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