heatmap and histogram need some improvements, histogram is currently broken after #78

• heatmaps for data larger than 8192, as discussed before this is a WGPU limitation. Need to stitch together multiple pygfx.Image world objects
• histogram is broken because histogram "data" is different from the graphic data. I think we should subclass DataFeature to make one specific to histograms, and it could handle things like each bin and its values. We could add a method to HistogramGraphic to change the scale_factor for displaying it. We might also need to subclass ColorFeature for histograms so that indexing sets the color of each bin.