gles2: Use GL_ES instead of __VERSION__ for setting precision

Mesa has started getting picky about specifying the precision for
floating types in the fragment shader. We already have a default
precision specifier in all the fragment shaders but apparently this
wasn't working because it is only used when the __VERSION__ define is
100 and Mesa is reporting 110. Regardless of whether Mesa is doing the
right thing or not I think it makes sense to use GL_ES instead of
__VERSION__ because we will also need the precision specifier if we
start requesting GLSL 3.0. The GLES specification explictly states
that GL_ES will only be defined for GLES and this is similar to what
the internal meta shaders do in Mesa.

http://cgit.freedesktop.org/mesa/mesa/commit/?id=cabd45773b58d6aa482

Reviewed-by: Robert Bragg <robert@linux.intel.com>