Add demo shader and spinning colored cube

src/app.d

@@ -3,10 +3,17 @@ import sdl;
 import gl;
 import glad.gl.loader;
 import gltk;
+import gl3n.linalg;
 
 enum int WIDTH = 800;
 enum int HEIGHT = 600;
 
+alias ColorShader = Program!(["view": "Matrix4fv"], ["position": 0, "color": 1]);
+ColorShader color_shader;
+Array spinny_vao;
+GLuint program_id;
+mat4 view_matrix;
+
 void init()
 {
     glActiveTexture(GL_TEXTURE0);
@@ -15,12 +22,65 @@ void init()
     glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
     glClearColor (1.0, 0.6, 0.0, 0.0);
     glViewport(0, 0, WIDTH, HEIGHT);
+
+    spinny_vao = new Array();
+    spinny_vao.bind();
+    string color_program_src = `
+#version 450
+
+uniform mat4 view;
+
+vertex:
+    in vec2 position;
+    in vec4 color;
+    out vec4 color_i;
+    void main(void)
+    {
+        gl_Position = view * vec4(position, 0.0, 1.0);
+        color_i = color;
+    }
+
+fragment:
+    in vec4 color_i;
+    out vec4 frag_color;
+    void main(void)
+    {
+        frag_color = color_i;
+    }`;
+    color_shader = new ColorShader(color_program_src);
+    color_shader.use();
+
+    float[] vertices = [0.4, 0.4, -0.4, 0.4, -0.4, -0.4, 0.4, -0.4];
+    float[] colors = [
+        1.0, 0.0, 0.0, 1.0,
+        0.0, 1.0, 0.0, 1.0,
+        0.0, 0.0, 1.0, 1.0,
+        1.0, 1.0, 1.0, 1.0];
+    ushort[] indices = [0, 1, 2, 3];
+    Buffer vbo = new Buffer(vertices);
+    vbo.bind();
+    glEnableVertexAttribArray(0);
+    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, null);
+    Buffer cbo = new Buffer(colors);
+    cbo.bind();
+    glEnableVertexAttribArray(1);
+    glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, null);
+    ElementBuffer ibo = new ElementBuffer(indices);
+    ibo.bind();
 }
 
 void display(SDL_Window * window)
 {
     glClear(GL_COLOR_BUFFER_BIT);
 
+    color_shader.use();
+    spinny_vao.bind();
+    view_matrix.make_identity();
+    view_matrix.scale(HEIGHT / cast(float)WIDTH, 1.0, 1.0);
+    view_matrix.rotatez(SDL_GetTicks() / 500.0);
+    color_shader.set_view(view_matrix.value_ptr);
+    glDrawElements(GL_TRIANGLE_FAN, 4, GL_UNSIGNED_SHORT, null);
+
     SDL_GL_SwapWindow(window);
 }