Position of watermark text now calculated in polar coordinates

Watermark text will now appear to rotate around the page centre, rather than at the bottom corner of the text element.

src/js/logic/add-watermark-page.ts

@@ -178,8 +178,8 @@ async function addWatermark() {
                 const textWidth = watermarkAsset.widthOfTextAtSize(text, fontSize);
 
                 page.drawText(text, {
-                    x: (width - textWidth) / 2,
-                    y: height / 2,
+                    x: (width / 2) - ((textWidth / 2) * Math.cos(angle * Math.PI / 180)),
+                    y: (height / 2) - ((textWidth / 2) * Math.sin(angle * Math.PI / 180)),
                     font: watermarkAsset,
                     size: fontSize,
                     color: rgb(textColor.r, textColor.g, textColor.b),