Offer to save file when closing if it is unsaved.

Don't hold all existing system fonts open.

On Windows, this exhausts file descriptors and everything (e.g.
opening and saving files) breaks.

Only generate split triangles when exporting if needed.

Before this commit, when exporting a vector file without the shaded
model shown, or similarly when using formats that we do not export
the mesh to, we still generate (and then discard) the mesh in paint
order. This is a waste of time.

CMake: properly invoke buildsystems of vendored libraries.

Before this commit, running "make" would build e.g. zlib's tests
and fail because they are not designed to work when zlib is included
as a subproject.

After this commit, EXCLUDE_FROM_ALL, which I did not know about,
prevents any actions from being done by default in these subprojects.

Don't crash after right-clicking on derived entities.

Reverse arrows on an angle constraint when the label is outside.

PDF, EPS: export stippled lines.

Update README: update prerequisites for OS X.

DXF: export entities, dimensions and comments on separate layers.

Only consider selected entities, when any, when doing Zoom to Fit.

Scoped "Zoom to Fit" is convenient for working on large models.
I (whitequark) have considered a separate shortcut, but its
usefulness is unclear and in any case it can be easily added
if desired.