Always fill the mask with the backdrop color

It fixes #18956.

In the patch #18029, for performance reasons and because I thought it was useless, I deliberately chose to not fill the mask
with the backdrop color when it's full black: it was a bad idea.
So in this patch we always add the backdrop color to the mask.

src/display/canvas.js

@@ -1498,6 +1498,7 @@ class CanvasGraphics {
     let maskY = layerOffsetY - maskOffsetY;
 
     if (backdrop) {
+      const backdropRGB = Util.makeHexColor(...backdrop);
       if (
         maskX < 0 ||
         maskY < 0 ||
@@ -1511,24 +1512,22 @@ class CanvasGraphics {
         );
         const ctx = canvas.context;
         ctx.drawImage(maskCanvas, -maskX, -maskY);
-        if (backdrop.some(c => c !== 0)) {
-          ctx.globalCompositeOperation = "destination-atop";
-          ctx.fillStyle = Util.makeHexColor(...backdrop);
-          ctx.fillRect(0, 0, width, height);
-          ctx.globalCompositeOperation = "source-over";
-        }
+        ctx.globalCompositeOperation = "destination-atop";
+        ctx.fillStyle = backdropRGB;
+        ctx.fillRect(0, 0, width, height);
+        ctx.globalCompositeOperation = "source-over";
 
         maskCanvas = canvas.canvas;
         maskX = maskY = 0;
-      } else if (backdrop.some(c => c !== 0)) {
+      } else {
         maskCtx.save();
         maskCtx.globalAlpha = 1;
         maskCtx.setTransform(1, 0, 0, 1, 0, 0);
         const clip = new Path2D();
         clip.rect(maskX, maskY, width, height);
         maskCtx.clip(clip);
         maskCtx.globalCompositeOperation = "destination-atop";
-        maskCtx.fillStyle = Util.makeHexColor(...backdrop);
+        maskCtx.fillStyle = backdropRGB;
         maskCtx.fillRect(maskX, maskY, width, height);
         maskCtx.restore();
       }

test/pdfs/.gitignore

@@ -679,3 +679,4 @@
 !issue18036.pdf
 !issue18894.pdf
 !bug1922766.pdf
+!issue18956.pdf

test/test_manifest.json

@@ -10734,5 +10734,12 @@
     "type": "eq",
     "link": true,
     "talos": false
+  },
+  {
+    "id": "issue18956",
+    "file": "pdfs/issue18956.pdf",
+    "md5": "73e8cd32bd063e42fcc4b270c78549b1",
+    "rounds": 1,
+    "type": "eq"
   }
 ]