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/apps/reader/modules/readerdogear.lua

@@ -35,7 +35,6 @@ function ReaderDogear:setupDogear(new_dogear_size)
             dimen = Geom:new{w = Screen:getWidth(), h = self.dogear_size},
             ImageWidget:new{
                 file = "resources/icons/dogear.png",
-                alpha = true,
                 width = self.dogear_size,
                 height = self.dogear_size,
             }