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 https://github.com/libvips/libvips/issues/1174