add glyph fallbacks

if glyph is not found in specified fontface, fallback fonts are checked
on by one until a glyph is found.

frontend/ui/font.lua

@@ -31,6 +31,11 @@ Font = {
 		-- font for info messages
 		infofont = "droid/DroidSans.ttf",
 	},
+	fallbacks = {
+		[1] = "freefont/FreeSans.ttf",
+		[2] = "droid/DroidSans.ttf",
+		[3] = "droid/DroidSansFallback.ttf",
+	},
 
 	fontdir = os.getenv("FONTDIR") or "./fonts",
 

frontend/ui/rendertext.lua

@@ -22,6 +22,16 @@ function getGlyph(face, charcode, bgcolor, fgcolor)
 		return glyph[1]
 	end
 	local rendered_glyph = face.ftface:renderGlyph(charcode, bgcolor, fgcolor)
+	if face.ftface:checkGlyph(charcode) == 0 then
+		for index, font in pairs(Font.fallbacks) do
+			DEBUG("fallback to font", font)
+			local fb_face = Font:getFace(font, face.size)
+			if fb_face.ftface:checkGlyph(charcode) ~= 0 then
+				rendered_glyph = fb_face.ftface:renderGlyph(charcode, bgcolor, fgcolor)
+				break
+			end
+		end
+	end
 	if not rendered_glyph then
 		DEBUG("error rendering glyph (charcode=", charcode, ") for face", face)
 		return