update

assets/shaders/2d-solid.fragment.glsl

@@ -1,10 +1,8 @@
 #version 330 core
 out vec4 frag_color;
 
-uniform vec3 color;
-
-in vec2 uv;
+in vec4 fragment_color;
 
 void main() {
-    frag_color = vec4(color, 0.0);
+    frag_color = fragment_color;
 };

assets/shaders/2d-solid.vertex.glsl

@@ -0,0 +1,29 @@
+#version 330 core
+layout (location = 0) in vec2 p0;
+layout (location = 1) in vec2 p1;
+layout (location = 2) in vec4 color;
+
+uniform mat4 projection;
+
+out vec4 fragment_color;
+
+const vec2 rectangle_vertices[4] = vec2[](
+    vec2(-1, -1),
+    vec2(-1,  1),
+    vec2( 1, -1),
+    vec2( 1,  1)
+);
+
+void main() {
+    vec2 dest_half_size = (p1 - p0) / 2;
+    vec2 dest_center = (p1 + p0) / 2;
+    vec2 dest_position = rectangle_vertices[gl_VertexID] * dest_half_size + dest_center;
+
+    gl_Position = projection * vec4(
+        dest_position,
+        0,
+        1
+    );
+
+    fragment_color = color;
+}