glsl: Allow default precision qualifiers to be set for sampler types.

From GLSL ES 3.00 section 4.5.4 ("Default Precision Qualifiers"):

    "The precision statement

        precision precision-qualifier type;

    can be used to establish a default precision qualifier. The type
    field can be either int or float or any of the sampler types, and
    the precision-qualifier can be lowp, mediump, or highp."

GLSL ES 1.00 has similar language.  GLSL 1.30 doesn't allow precision
qualifiers on sampler types, but this seems like an oversight (since
the intention of including these in GLSL 1.30 is to allow
compatibility with ES shaders).

Previously, Mesa followed GLSL 1.30 and only allowed default precision
qualifiers to be set for float and int.  This patch makes it follow
GLSL ES rules in all cases.

NOTE: This is a candidate for stable branches.