glsl: Allow precision qualifiers for sampler types

glsl: Allow precision qualifiers for sampler types

GLSL 1.30 doesn't allow precision qualifiers on sampler types,
but in GLSL ES, sampler types are also allowed. This seems like
an oversight (since the intention of including these in GLSL 1.30
is to allow compatibility with ES shaders).

Currently, Mesa allows "default" precision qualifiers to be set for
sampler types in GLSL (commit d5948f2). This patch makes it follow
GLSL ES rules and also allow declaring sampler variables with a
precision qualifier in GLSL 1.30 (and later). e.g.
uniform lowp sampler2D sampler;

This fixes a shader compilation error in Khronos OpenGL conformance
test "depth_texture_mipmap".

V2: Update comments.
Signed-off-by: Ian Romanick <idr@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <idr@lists.freedesktop.org>
Cc: <mesa-stable@lists.freedesktop.org>