remove rescaleByDPI since it's not reversible

alternatively we store the original size before scaling by screen DPI
and use that value when needed.

frontend/ui/device/screen.lua

@@ -171,10 +171,6 @@ function Screen:scaleByDPI(px)
     return math.ceil(px * self:getDPI()/167)
 end
 
-function Screen:rescaleByDPI(px)
-    return math.ceil(px * 167/self:getDPI())
-end
-
 function Screen:getRotationMode()
     return self.cur_rotation_mode
 end

frontend/ui/font.lua

@@ -54,6 +54,8 @@ function Font:getFace(font, size)
         font = self.cfont
     end
 
+    -- original size before scaling by screen DPI
+    local orig_size = size
     local size = Screen:scaleByDPI(size)
 
     local face = self.faces[font..size]
@@ -72,7 +74,7 @@ function Font:getFace(font, size)
         self.faces[font..size] = face
     --DEBUG("getFace, found: "..realname.." size:"..size)
     end
-    return { size = size, ftface = face, hash = font..size }
+    return { size = size, orig_size = orig_size, ftface = face, hash = font..size }
 end
 
 function Font:_readList(target, dir, effective_dir)

frontend/ui/rendertext.lua

@@ -73,8 +73,8 @@ function RenderText:getGlyph(face, charcode, bold, bgcolor, fgcolor)
     local rendered_glyph = face.ftface:renderGlyph(charcode, bgcolor, fgcolor, bold)
     if face.ftface:checkGlyph(charcode) == 0 then
         for index, font in pairs(Font.fallbacks) do
-            -- rescale face size by DPI since it will be scaled in getFace again
-            local fb_face = Font:getFace(font, Screen:rescaleByDPI(face.size))
+            -- use original size before scaling by screen DPI
+            local fb_face = Font:getFace(font, face.orig_size)
             if fb_face.ftface:checkGlyph(charcode) ~= 0 then
                 rendered_glyph = fb_face.ftface:renderGlyph(charcode, bgcolor, fgcolor, bold)
                 --DEBUG("fallback to font", font)