This gives what is to me smoother and more predictable snapping motion with
respect to the current mouse position.

For example-- imagine the grid cell size is 10 and the selected object under
the mouse already has its top left corner aligned with vertical grid line #2.

If I begin dragging that object to the left, within a single pixel of motion
it will snap to vertical grid line #1 which is 9 pixels away from the
object's current position.

With this commit, the user must drag the object cellsize / 2 pixels before it
snaps to a new position.

To me this seems smoother as it rounds to the nearest grid line rather than
to the grid line with the smallest coordinate value.

We also keep a partially transparent version of a grid for editmode. This
allows us to keep editmode visually distinct from runmode, even if snap-to-grid
isn't turned on.

If needed it's pretty easy to make the following changes:

* set the "big" cell size to something that isn't 100 (but I'm not sure how
  much control over the grid we actually want to give users-- e.g., setting
  the big cell to something for which the small cell isn't a factor
* allow snap-to-grid to be set independently for each canvas. (But again,
  what would be the use case for this?

Sam Thursfield authored Sep 16, 2020 and Albert Gräf committed Oct 14, 2020

This causes tar_em_up.sh to download an older version of nw.js suitable
for OSX 10.8.

It was accidentally removed by me in 4bffb36f.

Disable zexy/lpt on Windows for now, it doesn't work there at present and causes a segfault in the regression tests.