Support UTF-8 paths on Windows

Faceplates aren't loading if the RACK_USER_DIR has non-ASCII
characters in it. See:
https://community.vcvrack.com/t/rarebreeds-eugene-not-working/20266

Resolves issue #4.

src/OrbitsConfig.cpp

@@ -1,8 +1,55 @@
-#include <fstream>
-
 #include "OrbitsConfig.hpp"
 #include "plugin.hpp"
 
+static json_t *orbits_json_load_file(const char *path, size_t flags, json_error_t *error)
+{
+        FILE *fp = std::fopen(path, "rb");
+        json_t *json = json_loadf(fp, flags, error);
+        if (fp)
+        {
+                std::fclose(fp);
+        }
+        return json;
+}
+
+static std::string read_file_contents(const std::string& filename)
+{
+        // Read a file into a string without using ifstream.
+        // MINGW doesn't have an ifstream that supports being constructed
+        // from a wide string, which is required on Windows for opening
+        // paths containing UTF8 characters.
+        FILE* file = std::fopen(filename.c_str(), "rb");
+        if (file == NULL)
+        {
+                return "";
+        }
+
+        fseek(file, 0, SEEK_END);
+        long ftell_result = ftell(file);
+        fseek(file, 0, SEEK_SET);
+
+        if (ftell_result <= 0)
+        {
+                std::fclose(file);
+                return "";
+        }
+
+        size_t file_size = (size_t) ftell_result;
+        std::vector<char> buffer(file_size + 1);
+        size_t bytes_read = fread(buffer.data(), 1, file_size, file);
+
+        std::fclose(file);
+
+        if (bytes_read != static_cast<size_t>(file_size))
+        {
+                return "";
+        }
+
+        buffer[file_size] = '\0';
+
+        return std::string(buffer.data());
+}
+
 OrbitsConfig::OrbitsConfig(std::string path)
 {
         m_path = path;
@@ -33,7 +80,7 @@ std::string OrbitsConfig::getSvg(std::string component, int theme)
 {
         json_error_t error;
         std::string path = asset::plugin(pluginInstance, m_path);
-        json_t *root = json_load_file(path.c_str(), 0, &error);
+        json_t *root = orbits_json_load_file(path.c_str(), 0, &error);
         json_t *themes = json_object_get(root, "themes");
         json_t *entry = json_array_get(themes, theme);
         json_t *obj = json_object_get(entry, component.c_str());
@@ -50,8 +97,7 @@ std::string OrbitsConfig::getSvg(std::string component)
 
 Vec OrbitsConfig::getPos(std::string component)
 {
-        std::ifstream ifs(getSvg("panel"));
-        std::string content((std::istreambuf_iterator<char>(ifs)), (std::istreambuf_iterator<char>()));
+        std::string content = read_file_contents(getSvg("panel"));
         size_t search = content.find("inkscape:label=\"components\"");
         if(search == std::string::npos)
         {
@@ -73,8 +119,7 @@ Vec OrbitsConfig::getPos(std::string component)
 
 Vec OrbitsConfig::getSize(std::string component)
 {
-        std::ifstream ifs(getSvg("panel"));
-        std::string content((std::istreambuf_iterator<char>(ifs)), (std::istreambuf_iterator<char>()));
+        std::string content = read_file_contents(getSvg("panel"));
         size_t search = content.find("inkscape:label=\"components\"");
         search = content.find("inkscape:label=\"" + component, search);
         float w = rFindFloatAttribute(content, "width", search);
@@ -86,7 +131,7 @@ std::string OrbitsConfig::getThemeName(int theme)
 {
         json_error_t error;
         std::string path = asset::plugin(pluginInstance, m_path);
-        json_t *root = json_load_file(path.c_str(), 0, &error);
+        json_t *root = orbits_json_load_file(path.c_str(), 0, &error);
         json_t *themes = json_object_get(root, "themes");
         json_t *entry = json_array_get(themes, theme);
         json_t *name = json_object_get(entry, "name");
@@ -106,7 +151,7 @@ int OrbitsConfig::getDefaultThemeId()
         int default_theme_id = 0;
         json_error_t error;
         std::string path = asset::plugin(pluginInstance, m_path);
-        json_t *root = json_load_file(path.c_str(), 0, &error);
+        json_t *root = orbits_json_load_file(path.c_str(), 0, &error);
         json_t *def = json_object_get(root, "default");
         const char *default_name = json_string_value(def);
         json_t *themes = json_object_get(root, "themes");
@@ -131,7 +176,7 @@ size_t OrbitsConfig::numThemes()
 {
         json_error_t error;
         std::string path = asset::plugin(pluginInstance, m_path);
-        json_t *root = json_load_file(path.c_str(), 0, &error);
+        json_t *root = orbits_json_load_file(path.c_str(), 0, &error);
         json_t *themes = json_object_get(root, "themes");
         size_t count = json_array_size(themes);
         json_decref(root);
@@ -142,7 +187,7 @@ std::array<uint8_t, 3> OrbitsConfig::getColour(std::string component, int theme)
 {
         json_error_t error;
         std::string path = asset::plugin(pluginInstance, m_path);
-        json_t *root = json_load_file(path.c_str(), 0, &error);
+        json_t *root = orbits_json_load_file(path.c_str(), 0, &error);
         json_t *themes = json_object_get(root, "themes");
         json_t *entry = json_array_get(themes, theme);
         json_t *obj = json_object_get(entry, component.c_str());