Implement vertex color attribute for GLTF and IQM (#1790)

Added a simple cube with vertex colors for testing both.

examples/models/models_gltf_model.c

@@ -21,7 +21,7 @@
 
 #include <stdlib.h>
 
-#define MAX_MODELS  6
+#define MAX_MODELS  7
 
 int main(void)
 {
@@ -30,7 +30,7 @@ int main(void)
     const int screenWidth = 800;
     const int screenHeight = 450;
 
-    InitWindow(screenWidth, screenHeight, "raylib [models] example - model animation");
+    InitWindow(screenWidth, screenHeight, "raylib [models] example - model");
 
     // Define the camera to look into our 3d world
     Camera camera = { 0 };
@@ -48,6 +48,7 @@ int main(void)
     model[3] = LoadModel("resources/gltf/BoxAnimated.glb");
     model[4] = LoadModel("resources/gltf/AnimatedTriangle.gltf");
     model[5] = LoadModel("resources/gltf/AnimatedMorphCube.glb");
+    model[6] = LoadModel("resources/gltf/vertex_colored_object.glb");
 
     int currentModel = 0;