feat: add vaapi path for video wallpapers on generic gpus

Plugins/ZShell/Wallpaper/shaders/wallpaper_trans_linear_blur.frag

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+// Linear blur crossfade, from gl-transitions.com
+// Author: gre (MIT)
+// Description: a crossfade that passes through a directional blur.
+//
+// Ported to ZShell's GLSL ABI (see wallpaper_trans_bowtie.frag): the version
+// header is prepended by WallpaperService::shaderSource, the fullscreen
+// triangle comes from wallpaper.vert, and getFromColor/getToColor apply the
+// crop + flip convention. `uv` is bottom-left origin per gl-transitions.
+//
+// Optional gl-transitions params are fixed here: intensity = 0.1.
+
+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 intensity = 0.1;
+const int passes = 6;
+
+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) {
+	vec4 c1 = vec4(0.0);
+	vec4 c2 = vec4(0.0);
+
+	float disp = intensity * (0.5 - distance(0.5, progress));
+	for (int xi = 0; xi < passes; xi++) {
+		float x = float(xi) / float(passes) - 0.5;
+		for (int yi = 0; yi < passes; yi++) {
+			float y = float(yi) / float(passes) - 0.5;
+			vec2 v = vec2(x, y);
+			c1 += getFromColor(uv + disp * v);
+			c2 += getToColor(uv + disp * v);
+		}
+	}
+	c1 /= float(passes * passes);
+	c2 /= float(passes * passes);
+	return mix(c1, c2, progress);
+}
+
+void main() {
+	vec2 uv = gl_FragCoord.xy / size;
+	outColor = transition(uv);
+}