fix various problems, thanks CppCheck :) (#1005)

* explained a bit more the core_window_letterbox example

* fixed a few 'ups' moments that could lead to mild head pain and time loss

src/shapes.c

@@ -1524,7 +1524,7 @@ static float EaseCubicInOut(float t, float b, float c, float d)
 // Get texture to draw shapes (RAII)
 static Texture2D GetShapesTexture(void)
 {
-    if (texShapes.id <= 0)
+     if (texShapes.id == 0) //this variable is an unsigned int, will never be negative
     {
 #if defined(SUPPORT_FONT_TEXTURE)
         texShapes = GetFontDefault().texture;           // Use font texture white character