gpir basic conditional support

[enunes]

Implement 'less-than' and 'select' alu opcodes. Tested with gbm-surface and the following silly vertex shader:

attribute vec3 positionIn;
void main()
{
    vec3 myvar = positionIn;
    if (positionIn.x > 0.1)
        myvar.y = 0.8;
    gl_Position = vec4(myvar, 1);
}

It causes the rightmost edge of the gbm-surface triangle to have its y-coordinate modified.