Is your feature request related to a problem? Please describe.
Render the Start event with the BPMN standard.

Describe the solution you'd like
Create a new MxGraph Shape.

BPMN specification
As the name implies, the Start Event indicates where a particular Process will start. In terms of Sequence Flows, the Start Event starts the flow of the Process, and thus, will not have any incoming Sequence Flows—no Sequence Flow can connect to a Start Event.

The Start Event shares the same basic shape of the Intermediate Event and End Event, a circle with an open center so that markers can be placed within the circle to indicate variations of the Event.

A Start Event is a circle that MUST be drawn with a single thin line (see Figure 10.70).
The use of text, color, size, and lines for a Start Event MUST follow the rules defined in “Use of Text, Color, Size, and Lines in a Diagram” on page 39 with the exception that:

• The thickness of the line MUST remain thin so that the Start Event can be distinguished from the Intermediate and End Events.