Give a more sensible default crop in the post image picker (related #39)

src/lib/images.ts

@@ -112,3 +112,19 @@ export async function compressIfNeeded(
     maxSize,
   })
 }
+
+export interface Dim {
+  width: number
+  height: number
+}
+export function scaleDownDimensions(dim: Dim, max: Dim): Dim {
+  if (dim.width < max.width && dim.height < max.height) {
+    return dim
+  }
+  let wScale = dim.width > max.width ? max.width / dim.width : 1
+  let hScale = dim.height > max.height ? max.height / dim.height : 1
+  if (wScale < hScale) {
+    return {width: dim.width * wScale, height: dim.height * wScale}
+  }
+  return {width: dim.width * hScale, height: dim.height * hScale}
+}