[db] fix design size scaling

As discussed in issue lualatex#398.

Ad futuram rei memoriam the gist of it:

- For the index, all values are scaled (decipoints * sp) / 10 *
  (7227 / 7200).

- The ``bp`` case (the default, OT-standard), needs no conversion
  because it matches how values are stored in the index.

- The ``pt`` case essentially reverts the bp→pt part of scaling
  done for the database by scaling the asked size by the same
  factor, i. e. by 7227 / 7200.

- The ``dd`` needs an extra 1238 / 1157.

Requesting a font at 10pt will then:

- ask for a size of 655360 for ``bp`` / default;
- ask for 657817 for ``pt``;
- ask for 703870 for ``dd``.

src/luaotfload-database.lua

@@ -1042,22 +1042,27 @@ local lookup_fontname = function (specification, name, style)
     return nil, nil
 end
 
-local design_size_dimension
-local set_size_dimension
+local design_size_dimension  --- scale asked size if not using bp
+local set_size_dimension     --- called from config
 do
 
-    --- cf. TeXbook p. 57
+    --- cf. TeXbook p. 57; the index stores sizes pre-scaled from bp to
+    --- sp. This allows requesting sizes we got from the TeX end
+    --- without further conversion. For the other options *pt* and *dd*
+    --- we scale the requested size as though the value in the font was
+    --- specified in the requested unit.
+
     local dimens = {
-        pt = function (v) return v                 end,
-        bp = function (v) return (v * 7227) / 7200 end,
-        dd = function (v) return (v * 1238) / 1157 end,
+        bp = false,
+        pt = function (v) return v * (7227 / 7200)                 end,
+        dd = function (v) return v * (7227 / 7200) * (1238 / 1157) end,
     }
 
     design_size_dimension = dimens.bp
 
     set_size_dimension = function (dim)
         local f = dimens [dim]
-        if f then
+        if f ~= nil then
             logreport ("both", 4, "db",
                        "Interpreting design sizes as %q, factor %.6f.",
                        dim, f (1.000000))
@@ -1135,7 +1140,10 @@ lookup_font_name = function (specification)
             askedsize = 0
         end
     end
-    askedsize = design_size_dimension (askedsize)
+
+    if design_size_dimension ~= false then
+        askedsize = design_size_dimension (askedsize)
+    end
 
     resolved, subfont = lookup_familyname (specification,
                                            name,
@@ -1350,9 +1358,16 @@ local load_font_file = function (filename, subfont)
     return ret
 end
 
+--- Design sizes in the fonts are specified in decipoints. For the
+--- index these values are prescaled to sp which is what we’re dealing
+--- with at the TeX end.
+
 local get_size_info do --- too many upvalues :/
     --- rawdata -> (int * int * int | bool)
 
+    local sp = 2^16        -- pt
+    local bp = 7227 / 7200 -- pt
+
     get_size_info = function (rawinfo)
         local design_size         = rawinfo.design_size
         local design_range_top    = rawinfo.design_range_top
@@ -1363,13 +1378,13 @@ local get_size_info do --- too many upvalues :/
                            or design_range_top    ~= 0 and design_range_top
 
         if fallback_size then
-            design_size         = ((design_size         or fallback_size) * 2^16) / 10
-            design_range_top    = ((design_range_top    or fallback_size) * 2^16) / 10
-            design_range_bottom = ((design_range_bottom or fallback_size) * 2^16) / 10
+            design_size         = ((design_size         or fallback_size) * sp) / 10
+            design_range_top    = ((design_range_top    or fallback_size) * sp) / 10
+            design_range_bottom = ((design_range_bottom or fallback_size) * sp) / 10
 
-            design_size         = (design_size         * 7227) / 7200
-            design_range_top    = (design_range_top    * 7227) / 7200
-            design_range_bottom = (design_range_bottom * 7227) / 7200
+            design_size         = design_size         * bp
+            design_range_top    = design_range_top    * bp
+            design_range_bottom = design_range_bottom * bp
 
             return {
                 design_size, design_range_top, design_range_bottom,