Extend support for tiled tiff files to all color formats

This commit extends the recently added support for tiled tiff images
of color index format to also cover grayscale and mono images. RGB
images were already covered, since the libtiff API used for those
handles tiled images transparently.

This commit also simplifies the tiff read() function by sharing
common code among the different formats.

[ChangeLog][TIFF] Extend support for tiled tiff files to all color formats

Change-Id: I13f94bbca65dd6a35384a415906034e245ad8b79
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>