Fix bug with heatmap series show and hide by changing plot options

[alodos]

There are some bugs while redraw heatmap plot series with changing show<->hide.
The series of heatmap is visible when hide after show by setting options.

    // Here init plot with showing heatmap series...
    //...
    // Then try hide heatmap (may be switch style to other, bubbles, only points and other)
    plot.getData()[0].heatmap.show = false;  // for current data
    plot.getOptions().series.heatmap.show = false;  // for new data if need setData()
    plot.setupGrid();
    plot.draw();

Also this pull request fixes bug with backImage load when opt.series.heatmap.backImage is not used, but after plot redrawing (with set new data) (new Image).src will be set to null.
The browser tries to GET image with url=src=null from server.

PS: Minimized javascripts also updated but simple, by hand. Need correct rebuild from sources.