[BUG] Tasks are no more rounded after applying custom styles

[tbouffard]

Describe the bug
BPMN tasks must be rendered with rounded corners per BPMN Specification.
bpmn-visualization enforces this, but in this demo, once the custom fill color is applied to tasks, the rounded corner are dropped, and the diagram display raw rectangles instead.

To Reproduce
Steps to reproduce the behavior:

1. Use code of commit ff8e970 and run the demo
2. Load an XES
3. The tasks displaying frequency data are displayed as raw rectangles

Expected behavior
The BPMN tasks should display rounded corners.

Screenshots

Frequency data

https://www.youtube.com/watch?v=qmWcnZ4Ffn0&t=22s

Conformance data

https://www.youtube.com/watch?v=qmWcnZ4Ffn0&t=66s

Environment

Desktop (please complete the following information):

• OS: Windows, Ubuntu 20.04
• Browser: Chrome 110.0, Firefox 110.0

Additional context

This is probably due to an issue in the code calling the mxGraph native function and should be fixed with #3 and #12.