Fix gtype for Lua5.1 on Windows

src/vips/cdefs.lua

@@ -4,7 +4,7 @@ local ffi = require "ffi"
 
 -- GType is an int the size of a pointer ... I don't think we can just use
 -- size_t, sadly
-if ffi.arch == "x64" then
+if ffi.arch == "x64" and ffi.os ~= "Windows" then
     ffi.cdef [[
         typedef uint64_t GType;
     ]]

src/vips/gvalue.lua

@@ -56,6 +56,9 @@ gvalue.blob_type = gobject_lib.g_type_from_name("VipsBlob")
 gvalue.band_format_type = gobject_lib.g_type_from_name("VipsBandFormat")
 gvalue.blend_mode_type = version.at_least(8, 6) and gobject_lib.g_type_from_name("VipsBlendMode") or 0
 
+gvalue.comparable_type = type(gvalue.gdouble_type) == "number" and
+    function(gtype) return tonumber(gtype) end or
+    function(gtype) return gtype end
 gvalue.to_enum = function(gtype, value)
     -- turn a string into an int, ready to be passed into libvips
     local enum_value
@@ -84,7 +87,7 @@ end
 
 gvalue.set = function(gv, value)
     local gtype_raw = gv.gtype
-    local gtype = tonumber(gtype_raw)
+    local gtype = gvalue.comparable_type(gtype_raw)
     local fundamental = gobject_lib.g_type_fundamental(gtype_raw)
 
     if gtype == gvalue.gbool_type then
@@ -156,7 +159,7 @@ end
 
 gvalue.get = function(gv)
     local gtype_raw = gv.gtype
-    local gtype = tonumber(gtype_raw)
+    local gtype = gvalue.comparable_type(gtype_raw)
     local fundamental = gobject_lib.g_type_fundamental(gtype_raw)
 
     local result

src/vips/voperation.lua

@@ -73,7 +73,7 @@ end
 voperation.set = function(self, name, flags, match_image, value)
     local vob = self:vobject()
     local gtype_raw = vob:get_typeof(name)
-    local gtype = tonumber(gtype_raw)
+    local gtype = gvalue.comparable_type(gtype_raw)
 
     -- if the object wants an image and we have a constant, imageize it
     --