glsl: Enable dFdx, dFdy, and fwidth by default in GLSL ES 3.00.

glsl: Enable dFdx, dFdy, and fwidth by default in GLSL ES 3.00.

Previously, we only exposed them in desktop GL or with:

   #extension GL_OES_standard_derivatives : enable

GLSL ES 3.00 includes these without an extension, so we need to expose
them by default.

Note that the above #extension line results in an error or desktop GL,
so we don't need to worry about this.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>