glsl: Fix pi/2 constant in acos built-in function

glsl: Fix pi/2 constant in acos built-in function

In single precision, 1.5707963 becomes 1.5707962513 which is too
small.  However, 1.5707964 becomes 1.5707963705 which is just right.
The value 1.5707964 is already used in asin.ir.

NOTE: This is a candidate for stable release branches.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Olivier Galibert <galibert@pobox.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>