meson: align gl backend option defaults with autotools
Disable gl backend by default, just like autotools does:
--enable-gl=[no/auto/yes]
Enable cairo's OpenGL surface backend feature
[default=no]
--enable-glesv2=[no/auto/yes]
Enable cairo's OpenGLESv2 surface backend feature
[default=no]
--enable-glesv3=[no/auto/yes]
Enable cairo's OpenGLESv3 surface backend feature
[default=no]