glsl: Unconditionally define GL_FRAGMENT_PRECISION_HIGH in ES2 shaders.

glsl: Unconditionally define GL_FRAGMENT_PRECISION_HIGH in ES2 shaders.

This is really supposed to be defined only if the driver supports highp
in the fragment shader - but all of our current ES2 implementations do.
So, just define it.  In the future, we'll need to add a flag to
gl_context and only define the macro if the flag is set.

"Fixes" freedesktop.org bug #31673.