verify that new WebGL extensions are disabled

Support for two new WebGL extensions was added during the ESR60 development cycle. We should verify that both are disabled via webgl.disable-extensions = true (which Tor Browser sets by default). See:

​https://bugzilla.mozilla.org/show_bug.cgi?id=1250077 ​https://developer.mozilla.org/en-US/docs/Web/API/WEBGL_compressed_texture_astc

​https://bugzilla.mozilla.org/show_bug.cgi?id=1325113 ​https://developer.mozilla.org/en-US/docs/Web/API/WEBGL_compressed_texture_s3tc_srgb