bugfixes: sprite darkening and long stereotypes
bugfixes: sprite darkening and long stereotypes

* during sprite generation, sprites are now correctly darkened.
  Previously, if a sprite had at least one line of all `F` pixels, the
  sprite's pixels were not shifted up to their darkest possible value
  when making the sprite transparent.
* long stereotype names that were split across multiple lines broke the
  creole bolding (i.e. `**[stereotype_name]**`). This bold effect is now
  fixed, and bolding is applied on each line of the stereotype name.

milo-minderbinder/AWS-PlantUML