only specify precision for fragment shader

Per discussion in #106, this should avoid unnecessarily reverting to mediump in the vertex shader.

Note, the GL_ES macro is built-in to GLSL-ES.

I tested this code on my Intel GPU with the open-source Mesa driver.  It works for both GLES2 and GLES3.  But I don't know if you might want feedback from those (@olliwang) developing on true GLES2 devices before merging.