add freetype-based text rendering and box labels

resources/shaders/text2d.fragment

@@ -0,0 +1,11 @@
+#version 330 core
+in vec2 TexCoord;
+out vec4 FragColor;
+
+uniform sampler2D uText;
+uniform vec3 uColor;
+
+void main() {
+    float a = texture(uText, TexCoord).r;  // glyph alpha in RED channel
+    FragColor = vec4(uColor, a);
+}