bugfix: imprecision in drawing

scenario:
  slowly press down mouse button and drag to draw a line
  release mouse button

Before this commit the point would jump just a little bit on release,
and points would go slightly to the left of where I expect.

Yet another thing it's hard to write an automated test for.

Manual_tests.md

@@ -12,6 +12,7 @@ record those here.
 * How the screen looks. Our tests use a level of indirection to check text and
   graphics printed to screen, but not the precise pixels they translate to.
     - where exactly the cursor is drawn to highlight a given character
+    - analogously, how a shape precisely looks as you draw it
 
 ### Other compromises
 

drawing.lua

@@ -126,6 +126,9 @@ function Drawing.draw_pending_shape(drawing, top, left,right)
   local function py(y) return Drawing.pixels(y, width)+top end
   local mx = Drawing.coord(pmx-left, width)
   local my = Drawing.coord(pmy-top, width)
+  -- recreate pixels from coords to precisely mimic how the drawing will look
+  -- after mouse_released
+  pmx,pmy = px(mx), py(my)
   local shape = drawing.pending
   if shape.mode == nil then
     -- nothing pending