Feature/improved signature element (#2489)

# Description

Please provide a summary of the changes, including relevant motivation
and context.

Closes #(issue_number)

## Checklist

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have performed a self-review of my own code
- [ ] I have attached images of the change if it is UI based
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] If my code has heavily changed functionality I have updated
relevant docs on [Stirling-PDFs doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
- [ ] My changes generate no new warnings
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

---------

Co-authored-by: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>
Co-authored-by: Reece Browne <reece@stirling.pdf>

src/main/resources/static/js/sign/signature-canvas.js

@@ -0,0 +1,85 @@
+const signaturePadCanvas = document.getElementById('drawing-pad-canvas');
+const signaturePad = new SignaturePad(signaturePadCanvas, {
+  minWidth: 1,
+  maxWidth: 2,
+  penColor: 'black',
+});
+
+function addDraggableFromPad() {
+  if (signaturePad.isEmpty()) return;
+  const startTime = Date.now();
+  const croppedDataUrl = getCroppedCanvasDataUrl(signaturePadCanvas);
+  console.log(Date.now() - startTime);
+  DraggableUtils.createDraggableCanvasFromUrl(croppedDataUrl);
+}
+
+function getCroppedCanvasDataUrl(canvas) {
+  let originalCtx = canvas.getContext('2d');
+  let originalWidth = canvas.width;
+  let originalHeight = canvas.height;
+  let imageData = originalCtx.getImageData(0, 0, originalWidth, originalHeight);
+
+  let minX = originalWidth + 1,
+    maxX = -1,
+    minY = originalHeight + 1,
+    maxY = -1,
+    x = 0,
+    y = 0,
+    currentPixelColorValueIndex;
+
+  for (y = 0; y < originalHeight; y++) {
+    for (x = 0; x < originalWidth; x++) {
+      currentPixelColorValueIndex = (y * originalWidth + x) * 4;
+      let currentPixelAlphaValue = imageData.data[currentPixelColorValueIndex + 3];
+      if (currentPixelAlphaValue > 0) {
+        if (minX > x) minX = x;
+        if (maxX < x) maxX = x;
+        if (minY > y) minY = y;
+        if (maxY < y) maxY = y;
+      }
+    }
+  }
+
+  let croppedWidth = maxX - minX;
+  let croppedHeight = maxY - minY;
+  if (croppedWidth < 0 || croppedHeight < 0) return null;
+  let cuttedImageData = originalCtx.getImageData(minX, minY, croppedWidth, croppedHeight);
+
+  let croppedCanvas = document.createElement('canvas'),
+    croppedCtx = croppedCanvas.getContext('2d');
+
+  croppedCanvas.width = croppedWidth;
+  croppedCanvas.height = croppedHeight;
+  croppedCtx.putImageData(cuttedImageData, 0, 0);
+
+  return croppedCanvas.toDataURL();
+}
+
+function isMobile() {
+  const userAgentCheck = /Mobi|Android|iPhone|iPad|iPod|Windows Phone|Opera Mini/i.test(navigator.userAgent);
+  const viewportCheck = window.matchMedia('(max-width: 768px)').matches;
+  return userAgentCheck || viewportCheck;
+}
+
+function getDeviceScalingFactor() {
+  return isMobile() ? 3 : 10;
+}
+
+function resizeCanvas() {
+  const ratio = Math.max(window.devicePixelRatio || 1, 1);
+  const additionalFactor = getDeviceScalingFactor();
+
+  signaturePadCanvas.width = signaturePadCanvas.offsetWidth * ratio * additionalFactor;
+  signaturePadCanvas.height = signaturePadCanvas.offsetHeight * ratio * additionalFactor;
+  signaturePadCanvas.getContext('2d').scale(ratio * additionalFactor, ratio * additionalFactor);
+
+  signaturePad.clear();
+}
+
+new IntersectionObserver((entries, observer) => {
+  if (entries.some((entry) => entry.intersectionRatio > 0)) {
+    resizeCanvas();
+  }
+}).observe(signaturePadCanvas);
+
+new ResizeObserver(resizeCanvas).observe(signaturePadCanvas);