Reduce the number of tiles rendered by TiledHeatmapMesh

[t20100]

Is your feature request related to a problem?

As it is, the TiledHeatmapMesh renders the current subsampled image layer according to the resolution, but also all lower layers: in case some tiles are not loaded yet, this "fills" the holes with lower resolution.

Requested solution or feature

This has two issues:

• It implies to render more tiles than needed
• The TiledHeatmap spreads over multiple layer in depth which can be cumbersome when adding more components in the VisCanvas (e.g., regions of interest shapes or a second image with transparency on top of it).

It should be possible to achieve the same "progressive" display with only one layer of tiles: The tiles that are not loaded yet can display the corresponding sub-region in the available tiles at lower resolution. This probably needs to adapt HeatmapMesh to pass texture coordinates in order to select the sub-region