stop putting button state in a global

Symptom: a test (test_click_to_create_drawing) started randomly failing
after I inserted a `return` 2 commits ago.

Cause: my tests call edit.draw, but button handlers only get cleared in
app.draw. So my tests weren't clearing button handlers, and every call
to edit.draw was accumulating states. Still unclear why those were going
to different state objects after the `return`, but anyway. I'm not going
to understand every last thing that happens when things go wrong, just
guarantee they can't go wrong. And the way to do that is to decentralize
button handlers to each state that receives them.

The State object in buttons.lua doesn't have to be Editor_state. It just
has to be some table that provides a Schelling Point for shared state.

button.lua

@@ -3,19 +3,23 @@
 -- If the handler returns true, it'll prevent any further processing of the
 -- event.
 
-Button_handlers = {}
-
 -- draw button and queue up event handlers
-function button(name, params)
+function button(State, name, params)
+  if State.button_handlers == nil then
+    State.button_handlers = {}
+  end
   love.graphics.setColor(params.color[1], params.color[2], params.color[3])
   love.graphics.rectangle('fill', params.x,params.y, params.w,params.h, 5,5)
   if params.icon then params.icon(params.x, params.y) end
-  table.insert(Button_handlers, params)
+  table.insert(State.button_handlers, params)
 end
 
 -- process button event handlers
-function propagate_to_button_handlers(x, y, mouse_button)
+function propagate_to_button_handlers(State, x, y, mouse_button)
-  for _,ev in ipairs(Button_handlers) do
+  if State.button_handlers == nil then
+    return
+  end
+  for _,ev in ipairs(State.button_handlers) do
     if x>ev.x and x<ev.x+ev.w and y>ev.y and y<ev.y+ev.h then
       if ev.onpress1 and mouse_button == 1 then
         return ev.onpress1()

edit.lua

@@ -128,6 +128,7 @@ function edit.fixup_cursor(State)
 end
 
 function edit.draw(State)
+  State.button_handlers = {}
   App.color(Text_color)
   assert(#State.lines == #State.line_cache)
   if not Text.le1(State.screen_top1, State.cursor1) then
@@ -151,7 +152,7 @@ function edit.draw(State)
       end
       if line.data == '' then
         -- button to insert new drawing
-        button('draw', {x=4,y=y+4, w=12,h=12, color={1,1,0},
+        button(State, 'draw', {x=4,y=y+4, w=12,h=12, color={1,1,0},
           icon = icon.insert_drawing,
           onpress1 = function()
                        Drawing.before = snapshot(State, line_index-1, line_index)
@@ -208,7 +209,7 @@ end
 function edit.mouse_pressed(State, x,y, mouse_button)
   if State.search_term then return end
 --?   print('press', State.selection1.line, State.selection1.pos)
-  if propagate_to_button_handlers(x,y, mouse_button) then
+  if propagate_to_button_handlers(State, x,y, mouse_button) then
     -- press on a button and it returned 'true' to short-circuit
     return
   end

main.lua

@@ -133,7 +133,6 @@ function App.filedropped(file)
 end
 
 function App.draw()
-  Button_handlers = {}
   edit.draw(Editor_state)
 end