improve data_length handling in jpg load

libjpeg uses unsigned ints for data length, so we must use size_t
everywhere.

fix make dist

we'd renamed a file

faster and more accurate bilinear

go to fixed point earlier in calculation of interpolation coefficients

faster and more accurate bilinear

go to fixed point earlier in calculation of interpolation coefficients

all tested and working

revised formula for in/out/dest-in/dest-out, for vector and scalar paths

see libvips#1301

final update for 8.8.0-rc1

revise changelog

had Klies' magickload fix in there twice

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