Properly open and work with large TIFF files.

Properly open and work with large TIFF files.

Any file between 2^31 and 2^32 bytes shouldn't have its size displayed
as a negative number. Viewing more than 32768 pixels in each dimension
is only a matter of using 64bit arithmetic when calculating allocation
sizes and pointers into that memory. Multiplying is always suspicious.

While there, make the error messages clearer about what went wrong.

Signed-off-by: Klaus Ethgen <Klaus@Ethgen.de>