Request a reflow when doing page zoom and only modify the scaling of the
WebView scene after the first root pipeline display list with the new
zoom is ready. In addition:
- store zoom limits in `Scale` types
- send `ViewportDetails` along with the display list so that we can
detect when the root pipeline scale is ready.
Testing: This is quite hard to test as it requires verification that
contents are zoomed appropriately at the right time.
Fixes: #38091.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>