glsl: Fix round64 conversion function

glsl: Fix round64 conversion function

Fix round64 function to handle round to nearest even cases specially
with positive and negative numbers with fraction part 0.5.

v2: 1) Simplify unused bits (Elie Tournier)

Fixes:
   KHR-GL45.gpu_shader_fp64.builtin.round_dvec2
   KHR-GL45.gpu_shader_fp64.builtin.round_dvec3
   KHR-GL45.gpu_shader_fp64.builtin.round_dvec4
   KHR-GL45.gpu_shader_fp64.builtin.roundeven_double
   KHR-GL45.gpu_shader_fp64.builtin.roundeven_dvec2
   KHR-GL45.gpu_shader_fp64.builtin.roundeven_dvec3
   KHR-GL45.gpu_shader_fp64.builtin.roundeven_dvec4

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Elie Tournier <elie.tournier@collabora.com>
Acked-by: Anuj Phogat <anuj.phogat@gmail.com>