add a rectangle shader

share/jes/shaders/rect.f.glsl

@@ -0,0 +1,11 @@
+#version 130
+
+/* Color */
+uniform vec4 color;
+
+out vec4 frag_color;
+
+void main(void)
+{
+    frag_color = color;
+}

share/jes/shaders/rect.v.glsl

@@ -0,0 +1,25 @@
+#version 130
+
+/* Viewport width and height */
+uniform ivec2 viewport_size;
+/* Position offset */
+uniform ivec2 position;
+/* Rectangle size */
+uniform ivec2 size;
+
+/* Vertex coordinates: x, y */
+in vec2 coords;
+
+/**
+ * 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 * size), 0, 1);
+}