feat: add vaapi path for video wallpapers on generic gpus

Plugins/ZShell/Wallpaper/shaders/wallpaper_trans_window_slices.frag

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+// Window slices wipe, from gl-transitions.com
+// Author: gre (MIT)
+// Description: the incoming image appears through vertical slices that widen.
+//
+// Ported to ZShell's GLSL ABI (see wallpaper_trans_bowtie.frag).
+// Optional gl-transitions params are fixed here: count = 10.0,
+// smoothness = 0.5.
+
+uniform sampler2D from;
+uniform sampler2D to;
+uniform vec2 size;
+uniform vec4 fromCrop;
+uniform vec4 toCrop;
+uniform float progress;
+uniform float fromFlip;
+uniform float toFlip;
+
+out vec4 outColor;
+
+const float count = 10.0;
+const float smoothness = 0.5;
+
+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) {
+	float pr = smoothstep(-smoothness, 0.0, uv.x - progress * (1.0 + smoothness));
+	float s = step(pr, fract(count * uv.x));
+	return mix(getFromColor(uv), getToColor(uv), s);
+}
+
+void main() {
+	vec2 uv = gl_FragCoord.xy / size;
+	outColor = transition(uv);
+}