Implement double frame buffering

shaders/pp_frag.glsl

@@ -0,0 +1,18 @@
+#version 330 core
+
+struct Material {
+  sampler2D diffuse1;
+  sampler2D specular1;
+  float shininess;
+};
+
+in vec2 uv_coords;
+
+uniform Material material;
+// uniform sampler2D image_texture;
+
+out vec4 color;
+
+void main() {
+  color = vec4(vec3(texture(material.diffuse1, uv_coords)), 1.0);
+}