fix memleak in magick6load

IM ExceptionInfo were not being freed correctly. This patch
adds a small wrapper function and uses it to allocate and free all IM
exception objects.

Tested with im 6.9 and gm 1.3.

See:

libvips/lua-vips#24

libvips#1203

Fix up vips_text()

Fixes two issues:

1. vips_text() in autofit mode could set the wrong DPI, since it set the
DPI in its own copy of the variable, but did not do a final update on
the DPI setting that FT uses for rendering.

2. vips_text() in autofit mode allocated a new context each time, rather
than reusing the context for that call. This caused a small memory leak.

See libvips#1174

fix temp file open

was not auto-deleteing on windows, needs a fallback on linux

see libvips#1155

add a nifti wrapper for vips7

so nip2 can now load nifti images

add a compat stub for vips_region_shrink()

to keep ABI compat with 8.6

make vips_object_get_args() return an int

since malloc can fail

make test-suite survive make dist

fix a buffer overflow in the tiff reader

see https://github.com/jcupitt/libvips/issues/1039

improve accuracy of vector path

fix make dist