specify precision for fragment shader

Fixes "0:3(14): error: no precision specified this scope for type `vec4'"

The default for the vertex shader is 'precision highp float',
but there is no such default for the fragment shader, see
4.5.3 "Default Precision Qualifiers" in GLSL_ES_Specification_1.0.17.pdf

trigles2 now works with  "OpenGL ES 3.0 Mesa 10.0.2", IF I have libgles2-mesa-dev installed at build time.