Multiple requests for the same image tile when reprojecting tile layers

[ahocevar]

Describe the bug
When raster reprojection is performed for a tile layer, source tiles that are needed for multiple reproj tiles are requested for each reproj tile, instead of being cached/shared.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://openlayers.org/en/latest/examples/reprojection-by-code.html
2. Enter '31287' in the search field and click "Search"
3. Open the network tab of your dev tools, zoom in to a high zoom level (e.g. 19), and see multiple requests for the same tile

Expected behavior
Source tiles are only requested once.