Tile server: alpha channel

Recoloring of tiles with color=ff0000 (red) parameter generates:

This is either because the original image (accessionDot0.png) does not have the alpha channel set correctly and uses opaque colors, or due to g2d.drawImage not averaging across all channels and the resulting image cannot be re-colored correctly.

Recoloring should only change the RGB channels from one color to another while Alpha channel needs to stay as-is.

Check org.genesys2.util.ColorUtil#changeColor if it correctly changes only the R,G,B channels. Alpha channel should not be modified.

There seems to be a problem with colors configured below, missing Alpha channel:

final int originalColor = MappingService.DEFAULT_TILE_COLOR.getRGB();
final int updatedColor = newColor.getRGB();