Verify first 4 bytes of TIFF file

Avoids Olympus Raw (ORF) false positive
Support BigTIFF, thanks @bgilbert
Expose ability to control libpng's row filter

Disabling adaptive filters can improve image size
cplusplus sort-of working

refs need fixing up
Merge pull request #190 from lovell/png-set-filter

Expose ability to control libpng's row filter
allow symbolic names for flags

you can now use numbers or names for GFlags values, eg.:

$ vips copy 50020484-00001.png x.png[filter=avg]
$ vips copy 50020484-00001.png x.png[filter=64]

was numbers only before
Merge pull request #187 from lovell/tiff-tightener-7.40

Verify first 4 bytes of TIFF file
new C++ interface works!

phew
package up new vips8 C++ API
better bigtiff detection

update Changelog and bump version
Merge remote-tracking branch 'origin/7.40'

Conflicts:
	ChangeLog
	configure.ac
oops
cleanups ready for proto gen
split to a separate branch
full cpp wrapper compiles
add some simple extras

.bandjoin(), .bandsplit(), .sin() etc etc
add make constant image
set of overloads for new cpp API

all the usual ones .. compiles, but untested
reformat
add more type conversions

so we automatically do int and double -> doublevec and intvec