add "text" shader

runtime/shaders/text.f.glsl

@@ -0,0 +1,15 @@
+#version 410
+
+/* Texture coordinate: s, t */
+in vec2 texture_coord_v;
+
+/* Texture unit */
+uniform sampler2D texture;
+uniform vec4 color;
+
+out vec4 frag_color;
+
+void main(void)
+{
+    frag_color = vec4(1, 1, 1, texture2D(texture, texture_coord_v).a) * color;
+}

runtime/shaders/text.v.glsl

@@ -0,0 +1,27 @@
+#version 410
+
+/* Viewport width and height */
+uniform ivec2 viewport_size;
+/* Position of lower left corner of glyph */
+uniform ivec2 position;
+
+/* Vertex coordinates: x, y, s, t */
+layout(location = 0) in vec4 coords;
+
+/* Output texture coordinate: s, t */
+out vec2 texture_coord_v;
+
+/**
+ * Map coordinates such that:
+ * (0 .. viewport_size.[xy]) => (-1.0 .. 1.0)
+ */
+vec2 map_to_screen(vec2 position)
+{
+    return 2.0 * position / viewport_size - 1.0;
+}
+
+void main(void)
+{
+    gl_Position = vec4(map_to_screen(vec2(position) + coords.xy), 0, 1);
+    texture_coord_v = coords.zw;
+}