mesa,glsl: Move round_to_even() from glsl to mesa/main (v2)

mesa,glsl: Move round_to_even() from glsl to mesa/main (v2)

Move round_to_even's definition to mesa/main so that _mesa_float_to_half()
can use it in order to eliminate rounding bias.

In additon to moving the fuction definition, prefix its name with "_mesa",
just as all other functions in mesa/main are prefixed.

v2: Fix Android build.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>