Expose this extension if the host GL is desktop or GLES 3.0+ or
GLES 2.0 with the extension present.

fwidth and friends are commonly used by many shaders, including Three.js.

Change-Id: I38001e43e25da54d5735e2c4fe6de729a6d262c9
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>