MatrixTransition.filterQuality doesn't outline performance implications if it is provided or null

The documentation for MatrixTransition.filterQuality says:

> The transform will be applied by re-rendering the child if filterQuality is null, otherwise it controls the quality of an ImageFilter.matrix applied to a bitmap rendering of the child.

But it is not clear when/how often the child is re-rendered if null, or what the performance implications of that are, or how setting it affects the layer structure. So it can be non-obvious to a reader when to set it and when not to.

This came up in [reviewing this PR which sets it to null for platform views](https://github.com/flutter/packages/pull/12565#discussion_r3867589016).

Start at the MatrixTransition.filterQuality API documentation and inspect the related implementation to establish what happens when the value is null versus provided. Update the documentation to explain when the child is re-rendered, the performance implications, and how the setting affects layer structure; done means readers can tell when to use each option.

Written by the indexing model from the issue text.