GLES spec 3.0 - 4.5.4 Default Precision Qualifiers

defines the default precisions as follow for the vertex shaders :

precision highp float;
precision highp int;
precision lowp sampler2D;
precision lowp samplerCube;

and for the fragment shaders :

precision mediump int;
precision lowp sampler2D;
precision lowp samplerCube;

Therefore we need to set the Sampler3D precision in both vertex & fragment
shaders.

This also reverts :

"conform: Disable 3D textures on GLES"
commit ea34b9d7b9ade78b04d4056feb0a13fce6f7c953.