Change Activity Diagram "Stop" and "End" symbols to fill using the diagram background color instead of always using white

[matthew16550]

Similar to #448, this PR removes Activity Diagram hard coded WHITE colors.

@startuml

skinparam ActivityEndColor red
skinparam ActivityStartColor red
skinparam BackgroundColor grey
skinparam Shadowing false

start
end
stop

@enduml

Before

After

[matthew16550]

This might be causing an NPE, converted PR to draft while I investigate.

[matthew16550]

Never mind about the NPE, it seems that my IntelliJ PlantUML plugin was being strange 😆 .

[arnaudroques]

Thanks again !