Fix translation in crop editor's pan gesture.

SignalMessaging/Views/ImageEditor/ImageEditorCropViewController.swift

@@ -516,16 +516,20 @@ class ImageEditorCropViewController: OWSViewController {
             owsFailDebug("Missing pinchTransform.")
             return
         }
-        guard let locationStart = gestureRecognizer.locationStart else {
+        guard let oldLocationView = gestureRecognizer.locationStart else {
             owsFailDebug("Missing locationStart.")
             return
         }
-        let locationNow = gestureRecognizer.location(in: self.clipView)
 
-        let locationUnitStart = self.locationUnit(forLocationInView: locationStart, transform: gestureStartTransform)
-        let locationUnitNow = self.locationUnit(forLocationInView: locationNow, transform: gestureStartTransform)
-        let locationUnitDelta = CGPointSubtract(locationUnitNow, locationUnitStart)
-        let newUnitTranslation = CGPointAdd(gestureStartTransform.unitTranslation, locationUnitDelta)
+        // The beauty of using an SRT (scale-rotate-translation) tranform ordering
+        // is that the translation is applied last, so it's trivial to convert
+        // translations from view coordinates to transform translation.
+        let viewBounds = clipView.bounds
+        let newLocationView = gestureRecognizer.location(in: self.clipView)
+        // Our (view bounds == canvas bounds) so no need to convert.
+        let translation = newLocationView.minus(oldLocationView)
+        let translationUnit = translation.toUnitCoordinates(viewSize: viewBounds.size, shouldClamp: false)
+        let newUnitTranslation = gestureStartTransform.unitTranslation.plus(translationUnit)
 
         updateTransform(ImageEditorTransform(outputSizePixels: gestureStartTransform.outputSizePixels,
                                          unitTranslation: newUnitTranslation,
@@ -600,17 +604,19 @@ class ImageEditorCropViewController: OWSViewController {
     }
 
     @objc public func rotate90ButtonPressed() {
-        rotateButtonPressed(angleRadians: CGFloat.pi * 0.5)
+        rotateButtonPressed(angleRadians: CGFloat.pi * 0.5, rotateCanvas: true)
     }
 
     @objc public func rotate45ButtonPressed() {
-        rotateButtonPressed(angleRadians: CGFloat.pi * 0.25)
+        rotateButtonPressed(angleRadians: CGFloat.pi * 0.25, rotateCanvas: false)
     }
 
-    private func rotateButtonPressed(angleRadians: CGFloat) {
-        // Invert width and height.
-        let outputSizePixels = CGSize(width: transform.outputSizePixels.height,
-                                      height: transform.outputSizePixels.width)
+    private func rotateButtonPressed(angleRadians: CGFloat, rotateCanvas: Bool) {
+        let outputSizePixels = (rotateCanvas
+            // Invert width and height.
+            ? CGSize(width: transform.outputSizePixels.height,
+            height: transform.outputSizePixels.width)
+        : transform.outputSizePixels)
         let unitTranslation = transform.unitTranslation
         let rotationRadians = transform.rotationRadians + angleRadians
         let scaling = transform.scaling