ProgressWidget: Simplify painting logic.

* Use paintRect and plain colors instead of lightenRect and a weird
  dimming factor.
* Update call sites to the new API
* Handle FP maths properly (i.e., floor coordinates & ceil dimensions at
  the latest possible time).
* Fix border handling in the fill bar (make sure we actually honor it
  when computin the x position, and that we won't overflow into it when
  computing the width).
* Update docs

frontend/apps/reader/modules/readerrolling.lua

@@ -1210,7 +1210,7 @@ function ReaderRolling:showEngineProgress(percent)
             self.engine_progress_widget = ProgressWidget:new{
                 width = w,
                 height = h,
-                rectdim = 1, -- we need pure B&W for refreshFast
+                fillcolor = Blitbuffer.COLOR_BLACK, -- we need pure B&W for refreshFast
                 percentage = percent,
                 margin_h = 0,
                 margin_v = 0,