Change vertex buffer handling

Previously vertex data was only in shaders. I have removed it from there
and put hardcoded vertex data into cpp source code.

shaders/shader.vert

@@ -1,20 +1,11 @@
 #version 450
 
+layout(location = 0) in vec2 inPosition;
+layout(location = 1) in vec3 inColor;
+
 layout(location = 0) out vec3 fragColor;
 
-vec2 positions[3] = vec2[](
-    vec2(0.0, -0.5),
-    vec2(0.5, 0.5),
-    vec2(-0.5, 0.5)
-);
-
-vec3 colors[3] = vec3[](
-    vec3(1.0, 0.0, 0.0),
-    vec3(0.0, 1.0, 0.0),
-    vec3(0.0, 0.0, 1.0)
-);
-
 void main() {
-    gl_Position = vec4(positions[gl_VertexIndex], 0.0, 1.0);
-    fragColor = colors[gl_VertexIndex];
+    gl_Position = vec4(inPosition, 0.0, 1.0);
+    fragColor = inColor;
 }