Custom Shader for Transition Effect

### Feature Description

Add option to input a custom shader for the transitioning effect.

### Use Case

I would like that the transition effect matches my window opening and closing shader.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

An option to select a shader file.

### Alternatives/Existing Solutions

_No response_

No files, tests, or entry points are identified; first trace how transition effects are implemented across the supported compositors. Define the shader-file selection and compatibility requirements, then verify that a user-provided shader can drive the transition for all compositors.

Written by the indexing model from the issue text.