TileExporter with ImageJ TIFF can lose pixel size and channel color information

[petebankhead]

Bug report

Describe the bug
When using the TileExporter, pixel size and channel color information can be lost.

To Reproduce
Use the TileExporter class to export fixed-size tiles as ImageJ TIFF, e.g. with this script applied to the LuCa sample image:

def path = buildPathInProject("tiles")
mkdirs(path)
def imageData = getCurrentImageData()
new TileExporter(imageData)
    .writeTiles(path)

Open the resulting TIFFs in ImageJ and see that no calibration is set.

Expected behavior
Channel colors are preserved, and the image properties are set (pixel size and origin).

Desktop (please complete the following information):

• OS: All
• QuPath Version: v0.5.1 (and earlier)

Additional context
Relates to #1503 but is not identical.

The relevant code is at

qupath/qupath-core/src/main/java/qupath/lib/images/writers/TileExporter.java

Line 786 in 0ab5d05

// TODO: This has disadvantages, in that it loses channel names & region info

The difficulty is guaranteeing that tiles have a fixed size, rather than risking an off-by-one error, when we can only define the region with an ImageServer and RegionRequest that can have an arbitrary downsample.