Add support for material color

The GL_DEPTH_TEST (in the Window.java) has been disabled (removed)
because in the 2D graphics we don't need that, even more it was making
alpha color-channel unusable.

app/src/main/resources/shaders/default.fs

@@ -1,8 +1,10 @@
 #version 330
 
+uniform vec4 objectColor;
+
 out vec4 fragColor;
 
 void main()
 {
-   fragColor = vec4(0.0, 0.5, 0.5, 1.0);
+   fragColor = objectColor;
 }