Custom backdrop-filters support

Context:

Custom filter functions are currently disabled for backdrop-filter. When a FilterFunction based on a custom filter definition is assigned to backdrop-filter, it is ignored at render time and the following warning is logged:

> Custom filters are not supported for backdrop-filter on element ''. Custom filters will be ignored.

They were disabled deliberately: applying a custom filter can run user code, and backdrop filters are evaluated during rendering, so user code could change the rendering state mid-render.

Steps to reproduce (from [Internal link]):

1. Create a new Unity project using the Universal 3D template
2. In the Hierarchy, create a Button (Right-click > UI Toolkit > Standard Elements > Button)
3. In the "Open UI stage" pop-up, press "Continue" and save the UXML file
4. With the Button selected, expand "Backdrop Filter" in the Inspector
5. Select "Custom function"

Actual results:

The custom filter is ignored and the "Custom filters are not supported for backdrop-filter" warning is thrown in the Console. The UI Authoring Inspector still exposes "Custom Function" as a selectable option (UI Builder hides it), so users can reach this unsupported state from the UI ([Internal link]).

Expected results:

Custom filter functions work with backdrop-filter the same way they do with filter, with no Console warning.

Proposed approach:

Instead of running the custom filter's user code during rendering, apply the filter settings in the update phase and cache the result; the renderer then only consumes the precomputed result, so no user code executes while rendering.

Related issues:

• [Internal link] — UI Authoring Inspector allows selecting Custom Function in Backdrop Filter, throwing the warning in the Console (reproduced with 6000.6.0b1, 6000.6.0b6, 6000.7.0a3). Once this ticket lands, the option can be exposed in both UI Authoring and UI Builder instead of being hidden/disabled.