Inform once about color rendering on supported devices (#3289)

frontend/ui/elements/screen_color_menu_table.lua

@@ -8,7 +8,7 @@ return {
     enabled_func = Screen.isColorScreen,
     checked_func = Screen.isColorEnabled,
     callback = function()
-        G_reader_settings:flipNilOrTrue("color_rendering")
+        G_reader_settings:saveSetting("color_rendering", not Screen.isColorEnabled())
         UIManager:broadcastEvent(Event:new("ColorRenderingUpdate"))
     end
 }

reader.lua

@@ -141,6 +141,18 @@ if Device:needsTouchScreenProbe() then
     Device:touchScreenProbe()
 end
 
+-- Inform once about color rendering on newly supported devices
+-- (there are some android devices that may not have a color screen,
+-- and we are not (yet?) able to guess that fact)
+if Device.hasColorScreen() and not G_reader_settings:has("color_rendering") then
+    -- enable it to prevent further display of this message
+    G_reader_settings:saveSetting("color_rendering", true)
+    local InfoMessage = require("ui/widget/infomessage")
+    UIManager:show(InfoMessage:new{
+        text = _("Documents will be rendered in color on this device.\nIf your device is grayscale, you can disable color rendering in the screen sub-menu for reduced memory usage."),
+    })
+end
+
 local exit_code = nil
 
 if ARGV[argidx] and ARGV[argidx] ~= "" then