when exporting to PDF
If an individual layout item needs rasterisation in order to
be exported correctly, it can now be individually rasterised
without forcing every other item to also be rasterised.
This allows exports to PDF keeping as much as possible as vectors,
e.g. a map with layer opacity won't force labels, scalebars, etc
to be rasterised too.
To accompany this, a new "Always export as vectors" checkbox
was added to layout properties. If checked, this will force
the export to keep items as vectors, even when it causes the
output to look different to layouts.
Fixes#7885
Allows for creating non-antialiased images from layouts. Note that
some layout item types do not correctly respect this setting, but
at least map items do and the API is in place for them to be
fixed later.
Fixes#9281
