SengokuRanceFont: Fall back to Gothic TTF for missing FNL glyphs

SengokuRanceFont.DLL falls back to MS Gothic for characters whose glyphs
are absent from the FNL font. Reproduce this behavior to enable Japanese
translations of mods based on the MangaGamer release of Sengoku Rance.

Add an HLL-local composite font that wraps each selected FNL font_size
together with a FreeType Gothic font_size. The composite font checks for
each glyph in the FNL font and delegates rendering and size to Gothic
when the glyph is missing.

Scale fallback glyphs down and adjust their vertical position so that
they align with the FNL text line.

src/hll/SengokuRanceFont.c

@@ -43,6 +43,105 @@ struct sr_text_properties {
 	struct text_style ts;
 };
 
+// A font_size that uses FNL first and Gothic for missing glyphs.
+struct sr_fallback_font_size {
+	struct font_size super;
+	struct font_size *fnl;
+	struct font_size *gothic;
+	struct sr_fallback_font_size *next;
+};
+
+#define SR_FALLBACK_FONT_SCALE 0.75f
+#define SR_FALLBACK_VERTICAL_SHIFT_RATIO 0.65f
+
+static struct sr_fallback_font_size *sr_fallback_sizes;
+
+static uint32_t sjis_code_to_unicode(uint32_t code)
+{
+	char sjis[] = { code >> 8, code, 0 };
+	const char *p = code > 0xff ? sjis : sjis + 1;
+	int unicode;
+	sjis_char2unicode(p, &unicode);
+	return unicode;
+}
+
+static bool sr_fallback_get_glyph(struct font_size *_size, struct glyph *glyph,
+		uint32_t code, enum font_weight weight)
+{
+	struct sr_fallback_font_size *size = (struct sr_fallback_font_size*)_size;
+	if (size->fnl->font->get_glyph(size->fnl, glyph, code, weight))
+		return true;
+
+	if (!size->gothic->font->get_glyph(size->gothic, glyph,
+			sjis_code_to_unicode(code), weight))
+		return false;
+	// Align the gothic glyph toward the bottom of the FNL line.
+	glyph->rect.y += size->fnl->y_offset - size->gothic->y_offset;
+	glyph->rect.y -= lroundf((size->fnl->size - size->gothic->size)
+		* SR_FALLBACK_VERTICAL_SHIFT_RATIO);
+	return true;
+}
+
+static float sr_fallback_size_char(struct font_size *_size, uint32_t code)
+{
+	struct sr_fallback_font_size *size = (struct sr_fallback_font_size*)_size;
+	float width = size->fnl->font->size_char(size->fnl, code);
+	if (width != 0.0f)
+		return width;
+	return size->gothic->font->size_char(size->gothic,
+			sjis_code_to_unicode(code));
+}
+
+static float sr_fallback_size_char_kerning(struct font_size *size, uint32_t code,
+		uint32_t code_next)
+{
+	struct sr_fallback_font_size *fallback = (struct sr_fallback_font_size*)size;
+	float width = fallback->fnl->font->size_char_kerning(fallback->fnl,
+			code, code_next);
+	if (width != 0.0f)
+		return width;
+	return fallback->gothic->font->size_char_kerning(fallback->gothic,
+			sjis_code_to_unicode(code), sjis_code_to_unicode(code_next));
+}
+
+static struct font sr_fallback_font = {
+	.charmap = CHARMAP_SJIS,
+	.get_glyph = sr_fallback_get_glyph,
+	.size_char = sr_fallback_size_char,
+	.size_char_kerning = sr_fallback_size_char_kerning,
+};
+
+static struct font_size *sr_get_fallback_size(struct font_size *fnl)
+{
+	// One wrapper per FNL size.
+	for (struct sr_fallback_font_size *size = sr_fallback_sizes; size;
+			size = size->next) {
+		if (size->fnl == fnl)
+			return &size->super;
+	}
+
+	struct sr_fallback_font_size *size = xcalloc(1, sizeof(*size));
+	size->super.size = fnl->size;
+	size->super.y_offset = fnl->y_offset;
+	size->super.font = &sr_fallback_font;
+	size->fnl = fnl;
+	size->gothic = gfx_font_get_size(FONT_GOTHIC,
+			fnl->size * SR_FALLBACK_FONT_SCALE);
+	size->next = sr_fallback_sizes;
+	sr_fallback_sizes = size;
+	return &size->super;
+}
+
+static void sr_prepare_text_style(struct text_style *ts)
+{
+	if (ts->font_size)
+		return;
+	struct font_size *size = gfx_font_get_size(ts->face, ts->size);
+	// Install the wrapper only on FNL text styles.
+	ts->font_size = size->font->charmap == CHARMAP_SJIS
+		? sr_get_fallback_size(size) : size;
+}
+
 static struct sr_text_properties default_text_properties = {
 	.x = 0.0,
 	.y = 0,
@@ -134,6 +233,7 @@ static void SengokuRanceFont_SetEdgeWidth(float w)
 
 static float SengokuRanceFont_GetTextWidth(struct string *str)
 {
+	sr_prepare_text_style(&current_text_properties.ts);
 	return gfx_size_text(&current_text_properties.ts, str->text);
 }
 
@@ -326,6 +426,7 @@ static float sp_text_draw(struct sact_sprite *sp, struct sr_text_properties *tp,
 	if (tp->ts.size < 8) {
 		tp->ts.size = 8;
 	}
+	sr_prepare_text_style(&tp->ts);
 
 	if (!sp->text.texture.handle) {
 		gfx_init_texture_rgba(&sp->text.texture, sp->rect.w, sp->rect.h, COLOR(0, 0, 0, 0));