Feat/transition anim

[Atan-D-RP4]

Intending to implement full animations for the do-screen-transition action with custom-shader support.
Relevant discussion: #1620

The use-case is that I wanted more effects for workspace switch while overview is not enabled.

Extending the existing switch animation does not make sense design-wise and this action existed so this felt like a pretty natural workaround.

260227_21h55m23s_recording.mp4

[Atan-D-RP4]

It kinda works.

260302_00h38m54s_recording.mp4

[Copilot]

Pull request overview

Implements configurable screen transition animations for the do-screen-transition action, including optional custom GLSL shader support to render the transition effect (useful for workspace switching effects outside overview).

Changes:

• Reworks ScreenTransition to be driven by Animation config (with delay support) and optionally render via a custom shader program.
• Adds shader plumbing for a new ScreenTransition program type, including prelude/epilogue fragments and config-driven compilation/loading.
• Extends configuration and docs to expose animations.screen-transition with custom-shader.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/ui/screen_transition.rs	Moves from hardcoded crossfade timing to configurable animation and adds shader-based rendering path with uniforms.
src/render_helpers/shaders/screen_transition_prelude.frag	Defines the uniform/varying interface exposed to custom screen transition shaders.
src/render_helpers/shaders/screen_transition_epilogue.frag	Provides the standardized main() wrapper calling screen_transition_color(...).
src/render_helpers/shaders/mod.rs	Adds ProgramType::ScreenTransition, storage, and compilation/loading of custom screen transition shaders.
src/niri.rs	Wires config reload + rendering callsite (incl. passing per-output mouse position) and uses new animation config when triggering transitions.
src/backend/winit.rs	Loads configured screen transition shader at backend init.
src/backend/tty.rs	Loads configured screen transition shader at backend init.
niri-config/src/animations.rs	Introduces ScreenTransitionAnim with anim and custom_shader, plus decoding/merging.
niri-config/src/lib.rs	Updates config tests/fixtures to include the new screen_transition animation defaults.
docs/wiki/examples/screen_transition_custom_shader.frag	Adds documented example shader + uniform interface description and sample effects.
docs/wiki/Configuration:-Animations.md	Documents screen-transition and custom-shader configuration.

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[Copilot]

The custom-shader example for screen-transition references uniforms/textures (niri_geo_to_tex_next, niri_tex_next) that are not provided by the screen transition shader interface (it only exposes niri_geo_to_tex and niri_tex_from). This example as written won’t compile and is misleading for users; please update it to sample from niri_tex_from via niri_geo_to_tex (and fade based on niri_clamped_progress).

Suggested change

	vec3 coords_tex_next = niri_geo_to_tex_next * coords_curr_geo;
	vec4 color = texture2D(niri_tex_next, coords_tex_next.st);
	vec3 coords_tex = niri_geo_to_tex * coords_curr_geo;
	vec4 color = texture2D(niri_tex_from, coords_tex.st);