Increased performance of GL dropdown renderItem

## What does this MR do?

- Fixes an issue where `renderItem` is called several times even when not required
- Increased performance when rendering dropdown items
  - When using the `gl_dropdown.renderItem` it cuts the render time in approx. half. The main part holding it back now is finding the input to work out if it selected or not.

## What are the relevant issue numbers?

Closes #21110

See merge request !6222