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 <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Olivier Galibert <e31c272bd8cade1f5feffeb9bc971daaca5a8315@pobox.com>
Reviewed-by: Paul Berry <de21b9f22e6dd0bdf7890875d6e07ef4967c44ce@gmail.com>

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