mesa/vbo: Support the ES 3.0 signed normalized scaling rules.

mesa/vbo: Support the ES 3.0 signed normalized scaling rules.

Traditionally, OpenGL has had two separate equations for converting from
signed normalized fixed-point data to floating point data.  One was used
primarily for vertex data, while the other was primarily for texturing
and framebuffer data.

However, ES 3.0 and GL 4.2 change this, declaring there's only one
equation to be used in all cases.  Unfortunately, it's the other one.

v2: Correctly convert 0b10 to -1.0, as pointed out by Chris Forbes.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>