refactor ImGUI code files

internal/igwrap/backend/glyph_ranges.go

@@ -0,0 +1,24 @@
+package backend
+
+import "github.com/inkyblackness/imgui-go/v4"
+
+var glyphRanges imgui.AllocatedGlyphRanges
+
+// GlyphRanges returns a custom-built glyph ranges set.
+//
+// The ImGUI context must be already initialized.
+func GlyphRanges() imgui.GlyphRanges {
+	if glyphRanges.GlyphRanges == 0 {
+		b := &imgui.GlyphRangesBuilder{}
+
+		b.AddExisting(imgui.CurrentIO().Fonts().GlyphRangesChineseFull())
+
+		// Greek
+		b.Add(0x0391, 0x03A1)
+		b.Add(0x03A3, 0x03FF)
+
+		glyphRanges = b.Build()
+	}
+
+	return glyphRanges.GlyphRanges
+}