require editor margins to be ints

Not directly relevant here, but forks of this project that permit
zooming can run into weird glitches if margins are not a whole number of
pixels.

I'd always assumed a type system that divided ints into floats was
strictly superior, but now I have experienced a situation where
requiring ints isn't just a compromise for the underlying CPU
implementation. Particularly since Lua's print() silently hides really
tiny fractions.

Manual_tests.md

@@ -46,6 +46,9 @@ Lua is dynamically typed. Tests can't patch over lack of type-checking.
 * Like any high-level language, it's easy to accidentally alias two non-scalar
   variables. I wish there was a way to require copy when assigning.
 
+* I wish I could require pixel coordinates to integers. The editor defensively
+  converts input margins to integers.
+
 * My test harness automatically runs `test_*` methods -- but only at the
   top-level. I wish there was a way to raise warnings if someone defines such
   a function inside a dict somewhere.

edit.lua

@@ -90,8 +90,8 @@ function edit.initialize_state(top, left, right, font_height, line_height)  -- c
     em = App.newText(love.graphics.getFont(), 'm'),  -- widest possible character width
 
     top = top,
-    left = left,
-    right = right,
+    left = math.floor(left),
+    right = math.floor(right),
     width = right-left,
 
     filename = love.filesystem.getUserDirectory()..'/lines.txt',  -- '/' should work even on Windows