code: private TileBitmap createBackgroundBitmap(RenderContext renderContext) {
          TileBitmap bitmap = this.graphicFactory.createTileBitmap(renderContext.rendererJob.tile.tileSize, renderContext.rendererJob.hasAlpha);
          renderContext.canvasRasterer.setCanvasBitmap(bitmap);
          if (!renderContext.rendererJob.hasAlpha) {
              renderContext.canvasRasterer.fill(renderContext.renderTheme.getMapBackgroundOutside());
          }
          return bitmap;
  
      }
docs: Draws a bitmap just with outside colour, used for bitmaps outside of map area.
  
  @param renderContext the RenderContext
  @return bitmap drawn in single colour.