glsl/builtin_variables: Add a precision to the builtins

glsl/builtin_variables: Add a precision to the builtins

All of the builtin variables mentioned in the GLSL ES spec and the
extensions include a precision declaration which is different
depending on what the variable is used for. This patch makes it set
the corresponding precision when creating the variable. This will make
a difference once we start using the precision information for
optimisation. Previously all of the builtin variables ended up with a
precision of NONE.

v2: Made gl_PointSize and gl_FragCoord highp since GLSL ES 3.00. Fixed
    gl_MaxViewPorts to always be highp. (Eric Anholt)

Reviewed-by: Eric Anholt <eric@anholt.net>