Implement fill opacity for shading patterns in the SVG back-end

In the PDF file from the issue below, the fill alpha (`ca`) is set
before drawing the circles using the `setGState` operator. Doing so
causes the global alpha to be set on the canvas' context for the canvas
back-end, but this was not handled in the SVG back-end. This patch fixes
that by taking the fill opacity into account when drawing shading
patterns in the same way as done elsewhere so it is only included if the
value is non-default.

Fixes #11812.

src/display/svg.js

@@ -1090,6 +1090,9 @@ if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
       rect.setAttributeNS(null, "width", x1 - x0);
       rect.setAttributeNS(null, "height", y1 - y0);
       rect.setAttributeNS(null, "fill", this._makeShadingPattern(args));
+      if (this.current.fillAlpha < 1) {
+        rect.setAttributeNS(null, "fill-opacity", this.current.fillAlpha);
+      }
       this._ensureTransformGroup().appendChild(rect);
     }