[db] tentatively reintroduce fallback for italic

Fix assignment of LM series fonts. Currently these are broken because of
borked typosub identifiers like “8oblique” that prevent exact name
matching and at the same time exclude matching the (usable) subfamily.
Introduce a heuristic based on the italic angle value that assigns
italic as a fallback in these cases.

Test: https://bitbucket.org/phg/lua-la-tex-tests/src/857c83ca98cb35153979a0613d3a742bfd93f834/lua/tla-names-3-lm.lua

src/luaotfload-database.lua

@@ -1511,7 +1511,6 @@ end
 local organize_styledata = function (metadata, rawinfo, info)
     local pfminfo   = metadata.pfminfo
     local names     = rawinfo.names
-
     return {
     --- see http://www.microsoft.com/typography/OTSPEC/features_pt.htm#size
         size            = get_size_info (rawinfo),
@@ -2553,6 +2552,7 @@ generate_filedata = function (mappings)
 end
 
 local bold_spectrum_low  = 501 --- 500 is medium, 900 heavy/black
+local normal_weight      = 400
 local bold_weight        = 700
 local normal_width       = 5
 
@@ -2606,12 +2606,18 @@ do
             treating weights > 500 as bold or allowing synonyms like
             “heavy”, “black”.
         --]]-- 
-        if width == normal_width and pfmweight == bold_weight then
-            --- bold spectrum matches
-            if italicangle == 0 then
-                return "b"
+        if width == normal_width then
+            if pfmweight == bold_weight then
+                --- bold spectrum matches
+                if italicangle == 0 then
+                    return "b"
+                end
+                return "bi"
+            elseif pfmweight == normal_weight then
+                if italicangle ~= 0 then
+                    return "i"
+                end
             end
-            return "bi"
         end
         return false
     end