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

[timvandermeij]

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.

[timvandermeij]

You can test this by opening the preview viewer below and running PDFViewerApplication.preferences.set('renderer', 'svg'); in the console. If you refresh the page and open the file from the issue, you'll see three circles with different opacities now.

[timvandermeij]

/botio-linux preview

[pdfjsbot]

From: Bot.io (Linux m4)

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[pdfjsbot]

From: Bot.io (Linux m4)

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Full output at http://54.67.70.0:8877/9cde8c7afef8b6a/output.txt

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• Viewer: http://54.67.70.0:8877/9cde8c7afef8b6a/web/viewer.html
• Viewer (ES5): http://54.67.70.0:8877/9cde8c7afef8b6a/es5/web/viewer.html

[Snuffleupagus]

Looks good to me, thanks!

Super tiny nit: Shouldn't it be ca, i.e. lower-case, in the commit message since the upper-case variant is stroke-related based on e.g.

pdf.js/src/display/canvas.js

Lines 1005 to 1011 in 3b615e4

	case "CA":
	this.current.strokeAlpha = state[1];
	break;
	case "ca":
	this.current.fillAlpha = state[1];
	this.ctx.globalAlpha = state[1];
	break;

This confused me for a second when reading the commit message, before looking at the actual code.

[timvandermeij]

Good point; I have amended the commit and PR message. Thanks!

[Snuffleupagus]

Good point; I have amended the commit and PR message.

Thank you; as an aside you do have to wonder why anyone thought it'd be a good idea to specify two separate operators with the same name and differing only in their casing...