show pixel size in the raster layer properties with 19 significant digits (fix #21023)

[alexbruy]

Description

Pixel size in the raster layer properties reported with 9 significat digits, this can be a source of the issues in case of very large datasets when pixes size value used to create new raster. With this PR we show pixel size with 19 significant digits to match GDAL's geotransform precision.

Fixes https://issues.qgis.org/issues/21023

Checklist

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