fix: View Hierarchy positioning is relative

[timfish]

It was not mentioned in the RFC but View Hierarchy positioning is expected to be relative to the parent window.

In Sentry, flutter projects were hard-coded to use absolute positioning but it was not possible to override this for other platforms.

I opened a PR to allow overriding this by including absolute in the rendering_system:

• https://github.com/getsentry/sentry/pull/83292

[timfish]

Another alternative might be to add a top level positioning property which can be relative or absolute and defaults to relative if not set?