shaped-texture: Ensure valid size after viewport changes

From the main commit:

When the texture size is invalidated using `invalidate_size()`, the new
size will only get calculated the next time `update_size()` is
called. This happens e.g. in `meta_shaped_texture_get_preferred_size()`
via `ensure_size_valid()`.

`update_size()` can chain up to `clutter_content_invalidate_size()`
as well as emitting a `size-changed` signal. If this happens during
layout, the result is a 'change the layout conditions during layout'
issue, causing heavy breakage in e.g. the Shell overview.

To fix this, expose `ensure_size_valid()` as API so callers can make
sure the texture has a valid size without creating redundant size
invalidations calls.

Note that if a buffer with a new size is attached we already trigger
`update_size()` explicitely, avoiding such situations.

Closes #1718 (closed)