feat: add lqip directive

Adds a low-quality-placeholder directive as discussed in #86

packages/core/src/transforms/lqip.ts

@@ -0,0 +1,28 @@
+import { getMetadata } from "../lib/metadata";
+import { TransformFactory } from "../types";
+
+export interface LqipOptions {
+    lqip: string
+}
+
+export const lqip: TransformFactory<LqipOptions> = (config, ctx) => {
+    const width = Math.log(1 + parseInt(config.lqip || '')) * 10
+    const quality = clamp(Math.log(1 + parseInt(config.lqip || '')) * 10, 0, 100)
+
+    if(!width || !quality) return
+
+    return function lqipTransform(image) {
+        const format = getMetadata(image, 'format') // needed for the quality directive
+
+        console.log(width,quality);
+
+        return image
+            .toFormat(format,{ quality: Math.floor(quality) })
+            .resize({ width: Math.floor(width) })
+    }
+}
+
+function clamp(num: number, min: number, max: number) {
+    const t = num < min ? min : num
+    return t > max ? max : t
+}