shell: Define COGL_ENABLE_EXPERIMENTAL_API and related defines globally

This allows us to use new Cogl APIs without having to define things in
every .c/.h file manually.
st: Adapt the mutter implementation for create_texture_material

This is the new API that uses set_null_texture instead of a dummy texture.
texture-cache: Remove FBO path for padding icons

GtkIconTheme now pads icons when it loads them to make sure that they
are square, so this code is effectively dead.
texture-cache: Port to modern Cogl
Port to cogl_texture_2d_new_from_data