feat: add vaapi path for video wallpapers on generic gpus

Plugins/ZShell/Wallpaper/shaders/wallpaper_trans_fade.frag

@@ -0,0 +1,39 @@
+// Fade (crossfade) transition, from gl-transitions.com.
+//
+// Ported to ZShell's own GLSL ABI (same contract as wallpaper_trans_bowtie.frag):
+// version/precision header is prepended by WallpaperService::shaderSource, the
+// fullscreen triangle comes from shaders/wallpaper.vert, and getFromColor /
+// getToColor sample with the crop + per-layer flip convention.
+//
+//   uniforms: samplers `from`/`to`, vec2 size, vec4 fromCrop, vec4 toCrop,
+//             float fromFlip, float toFlip, float progress (0..1).
+
+uniform sampler2D from;
+uniform sampler2D to;
+uniform vec2 size;
+uniform vec4 fromCrop;
+uniform vec4 toCrop;
+uniform float fromFlip;
+uniform float toFlip;
+uniform float progress;
+
+out vec4 outColor;
+
+vec4 getFromColor(vec2 uv) {
+	vec2 s = vec2(uv.x, fromFlip > 0.5 ? 1.0 - uv.y : uv.y);
+	return texture(from, fromCrop.xy + s * fromCrop.zw);
+}
+
+vec4 getToColor(vec2 uv) {
+	vec2 s = vec2(uv.x, toFlip > 0.5 ? 1.0 - uv.y : uv.y);
+	return texture(to, toCrop.xy + s * toCrop.zw);
+}
+
+vec4 transition(vec2 uv) {
+	return mix(getFromColor(uv), getToColor(uv), progress);
+}
+
+void main() {
+	vec2 uv = gl_FragCoord.xy / size;
+	outColor = transition(uv);
+}