GLSL: Make sampling operations have agnostic precision qualifiers for desktop.

Desktop defaults to highp for samplers, but this should not apply to the built-in
functions, so make it appy only to user declarations.

Test/spv.precisionNonESSamp.frag

@@ -0,0 +1,16 @@
+#version 450
+
+precision lowp sampler2D;
+precision lowp int;
+precision lowp float;
+
+uniform lowp sampler2D s;
+
+layout(location = 0) in lowp vec2 v;
+
+layout(location = 0) out lowp vec4 color;
+
+void main()
+{
+    color = texture(s, v);
+}