Tiles shouldn't be removed until new tiles are loaded

[dgarciabriseno]

Helioviewer's scaling/zooming has always depended on displaying tiles. Higher resolution tiles are shown when zoomed in and lower resolution tiles are shown when zoomed out.

When you zoom in and out, there are thresholds where helioviewer decides to get a tile at a higher/lower resolution. When you zoom in (or out) and the tiles are not available, you'll see black/empty tiles where the image is waiting to load. Depending on the server's load, it can take seconds for the tile to be loaded. We shouldn't remove the lower resolution tile until the higher resolution tile is loaded, this way there are gaps when zooming in or out.

This behavior can be seen in applications like google maps where zooming in will show a blurry low res image until the newer high res image is loaded.