Various blitting related cleanups (#4789)

* Fix the "Enable debug logging" checkbox so that it properly disables "Enable verbose debug logging" when it gets disabled
* Avoid asking ImageWidget for alpha-blending when it's not useful
* Make ImageWidget's alpha-blending code-path double-check that alpha-blending actually is needed, and avoid it if possible
* In the same vein, only do alpha-blending in textboxwidget when absolutely necessary
* Prefer color constants over the gray() method, ensuring that they're part of the eInk palette

Depends on https://github.com/koreader/koreader-base/pull/853

Fix #4774

frontend/ui/widget/progresswidget.lua

@@ -41,7 +41,7 @@ local ProgressWidget = Widget:new{
     bordersize = Screen:scaleBySize(1),
     bordercolor = Blitbuffer.COLOR_BLACK,
     bgcolor = Blitbuffer.COLOR_WHITE,
-    rectcolor = Blitbuffer.gray(0.7),
+    rectcolor = Blitbuffer.COLOR_DIM_GRAY,
     percentage = nil,
     ticks = nil,
     tick_width = Screen:scaleBySize(3),