Clean up of sandbox

* Added a statistics panel
* Added comments to `Sandbox.cpp`
* Moved utility functions and GUI code out of sandbox.cpp

BarinkEngine/graphics/shaders/fragment.shader

@@ -2,6 +2,8 @@
 
 out vec4 FragColor;
 
+uniform vec3 MatColour;
+
 void main(){
-	FragColor = vec4(0.5f, 0.5f, 0.0f , 1.0f);
+	FragColor = vec4(MatColour, 1.0f);
 }