Factory for delegating construction of bitmap images. World Wind shapes configured with a BitmapFactory construct
their bitmaps lazily, typically when the shape becomes visible on screen.
Returns the bitmap associated with this factory. This method may be called more than once and may be called
from a non-UI thread. Each invocation must return a bitmap with equivalent content, dimensions and
configuration. Any side effects applied to the World Wind scene by the factory must be executed on the main
thread.
The factory must not retain any reference to the returned bitmap and must not attempt to recycle the bitmap.