adding an Undo button to editor drawing

app/src/main/kotlin/com/simplemobiletools/gallery/pro/activities/EditActivity.kt

@@ -460,6 +460,10 @@ class EditActivity : SimpleActivity(), CropImageView.OnCropImageCompleteListener
             config.lastEditorBrushSize = it
             updateBrushSize(it)
         }
+
+        bottom_draw_undo.setOnClickListener {
+            editor_draw_canvas.undo()
+        }
     }
 
     private fun updateBrushSize(percent: Int) {

app/src/main/kotlin/com/simplemobiletools/gallery/pro/views/EditorDrawCanvas.kt

@@ -139,4 +139,14 @@ class EditorDrawCanvas(context: Context, attrs: AttributeSet) : View(context, at
         draw(canvas)
         return bitmap
     }
+
+    fun undo() {
+        if (mPaths.isEmpty()) {
+            return
+        }
+
+        val lastKey = mPaths.keys.lastOrNull()
+        mPaths.remove(lastKey)
+        invalidate()
+    }
 }