cpp: tiff: Move TileType out of TileInfo
tiletypefix
cpp: PixelBuffer: Add assignment operator
fixvariantassign
cpp: test: Update VariantPixelBuffer operator tests
cpp: tiff: Add field value caching and wrapper methods on IFD

Since it's not possible to use TIFFGetField when writing, tags
which need to be queried must be cached.
tiffwritefixes