Transparent SVG workspace images should show the configured node background color

[tschendel]

Problem

When an SVG image with transparent areas is used as a workspace image for a diagram node in Capella, the transparent areas do not show the configured node background color.

Instead, the transparent SVG areas show the diagram/canvas background. This makes it difficult to use transparent SVG symbols together with Capella/Sirius styling, because the configured background color of the diagram element is visually ignored.

Expected Behavior

If a node has a configured background color and an SVG workspace image is rendered on top of it, transparent areas of the SVG should show the node background color.

Rendering order should effectively be:

1. Paint the figure/node background color.
2. Render the SVG image above it.
3. Keep transparent SVG areas transparent, so the configured background color remains visible.

Current Behavior

Transparent SVG areas show the diagram/canvas background instead of the configured node background color.

Environment

Observed with:

[tschendel]

I also created a corresponding upstream Sirius issue because the affected rendering class appears to be part of Sirius Desktop:
"org.eclipse.sirius.diagram.ui.tools.api.figure.SVGFigure"

Sirius issue:
#737

This Capella issue can track the Capella user impact and the later consumption of the Sirius fix.