Store canvas data of different pages separately

This is only the proper way to ensure that the used canvas data matches
the context the text is being rendered to. Also, we need to clear the
canvas data (and cached styles) only if there *is* a forced redraw.

addon/doq.js

@@ -38,7 +38,7 @@ const DOQReader = {
   preferences: {},
   colorSchemes: [],
   readerTone: {},
-  canvasData: null,
+  canvasData: new WeakMap(),
   styleCache: new Map(),
   options: { autoReader: true, dynamicTheme: true },
   flags: { readerOn: false, isPrinting: false },
@@ -140,7 +140,7 @@ const DOQReader = {
   saveCanvas(ctx, method) {
     const cvs = ctx.canvas;
     if (cvs.isConnected && cvs.closest(".canvasWrapper"))
-      this.canvasData = ctx.getImageData(0, 0, cvs.width, cvs.height);
+      this.canvasData.set(ctx, ctx.getImageData(0, 0, cvs.width, cvs.height));
   },
 
   /* Return fill and stroke styles */
@@ -164,14 +164,15 @@ const DOQReader = {
     return newStyle;
   },
   getCanvasColor(ctx, tx, ty) {
-    if (!this.canvasData)
+    if (!this.canvasData.has(ctx))
       return null;
     const tfm = ctx.getTransform();
     let {x, y} = tfm.transformPoint({x: tx, y: ty});
     [x, y] = [x, y].map(Math.round);
-    const i = (y * this.canvasData.width + x) * 4;
-    const data = Array.from(this.canvasData.data.slice(i, i + 3));
-    return newColor(data.map(e => e / 255));
+    const canvasData = this.canvasData.get(ctx);
+    const i = (y * canvasData.width + x) * 4;
+    const rgb = Array.from(canvasData.data.slice(i, i + 3));
+    return newColor(rgb.map(e => e / 255));
   },
 
   /* Calculate a new style for given colorscheme and tone */
@@ -322,8 +323,6 @@ const DOQReader = {
       this.enableReader(e?.isTrusted);
     }
     this.updatePreference("tone", pick);
-    this.styleCache.clear();
-    this.canvasData = null;
   },
   forceRedraw() {
     const {pdfViewer, pdfThumbnailViewer} = window.PDFViewerApplication;
@@ -332,6 +331,8 @@ const DOQReader = {
       Object.values(store || {}).forEach(e => e.rebuild());
       return page;
     };
+    this.styleCache.clear();
+    this.canvasData = new WeakMap();
     pdfViewer._pages.filter(e => e.renderingState).map(rebuildAnnotations)
                     .forEach(e => e.reset());
     pdfThumbnailViewer._thumbnails.filter(e => e.renderingState)