feat: render per-vertex color triangle

- Add shader class for vertex/fragment program compilation
- Add vao class for vertex array object management
- Render RGB gradient triangle with interpolated vertex colors

cuber.cpp

@@ -2,20 +2,71 @@
 #include "GLFW/glfw3.h"
 
 #include "cbt/opengl/context.hpp"
+#include "cbt/opengl/shader.hpp"
+#include "cbt/opengl/buffer.hpp"
+#include "cbt/opengl/vao.hpp"
 
-#include <csignal>
-
-#include <asio.hpp>
-
+#include "asio.hpp"
 #include "glad/glad.h"
 
-auto main(int, char const*[]) -> int {
+auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
     auto ctx = cbt::opengl::context("cuber", 1280, 720);
 
     if (!ctx.valid()) {
         return 1;
     }
 
+    // compile shader
+    auto prog = cbt::opengl::shader();
+
+    const char* vert_src = R"glsl(
+        #version 410 core
+        layout(location = 0) in vec3 a_pos;
+        layout(location = 1) in vec3 a_color;
+        out vec3 v_color;
+        void main() {
+            gl_Position = vec4(a_pos, 1.0);
+            v_color = a_color;
+        }
+    )glsl";
+
+    const char* frag_src = R"glsl(
+        #version 410 core
+        in vec3 v_color;
+        out vec4 frag_color;
+        void main() {
+            frag_color = vec4(v_color, 1.0);
+        }
+    )glsl";
+
+    if (!prog.compile_vertex(vert_src) || !prog.compile_fragment(frag_src) || !prog.link()) {
+        return 1;
+    }
+
+    // vertex data: triangle with per-vertex colors
+    // red, green, blue corners
+    std::array<float, 18> vertices = {
+        -0.5f, -0.5f, 0.0f,  1.0f, 0.0f, 0.0f,
+         0.5f, -0.5f, 0.0f,  0.0f, 1.0f, 0.0f,
+         0.0f,  0.5f, 0.0f,  0.0f, 0.0f, 1.0f,
+    };
+
+    auto vbo = cbt::opengl::buffer(cbt::opengl::buffer_type::vertex);
+    vbo.upload(vertices.data(), vertices.size() * sizeof(float));
+
+    // bind VAO and configure vertex attributes
+    auto vao = cbt::opengl::vao();
+    vao.bind();
+    vbo.bind();
+    glEnableVertexAttribArray(0);
+    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), nullptr);
+    glEnableVertexAttribArray(1);
+    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float),
+                         reinterpret_cast<void*>(3 * sizeof(float)));
+    vbo.unbind();
+    vao.unbind();
+
+    // signal handling
     asio::io_context io;
     asio::signal_set signals(io, SIGINT, SIGTERM);
     bool quit = false;
@@ -29,6 +80,7 @@ auto main(int, char const*[]) -> int {
         while (io.poll()) {}
     };
 
+    // render loop
     while (!ctx.should_close()) {
         process_signals();
 
@@ -39,6 +91,12 @@ auto main(int, char const*[]) -> int {
         glClearColor(0.6f, 0.8f, 1.0f, 1.0f);
         glClear(GL_COLOR_BUFFER_BIT);
 
+        prog.use();
+        vao.bind();
+        glDrawArrays(GL_TRIANGLES, 0, 3);
+        vao.unbind();
+        prog.unuse();
+
         ctx.swap_buffers();
         ctx.poll_events();
     }