add freetype-based text rendering and box labels

resources/shaders/text2d.fragment

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+#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);
+}

resources/shaders/text2d.vertex

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+#version 330 core
+in vec2 vPosition;
+in vec2 vTexCoord;
+
+out vec2 TexCoord;
+
+uniform mat4 uProjection;
+
+void main() {
+    TexCoord = vTexCoord;
+    gl_Position = uProjection * vec4(vPosition.xy, 0.0, 1.0);
+}