add scale factor to obj shader and scale tank object up

assets/fs/shaders/obj_v.glsl

@@ -1,4 +1,6 @@
 
+uniform float scale;
+
 attribute vec3 pos;
 attribute vec3 normal;
 
@@ -7,7 +9,7 @@ varying vec3 normal_i;
 
 void main(void)
 {
-    gl_Position = gl_ModelViewProjectionMatrix * vec4(pos, 1);
+    gl_Position = gl_ModelViewProjectionMatrix * vec4(scale * pos, 1);
     pos_i = gl_Position.xyz;
     normal_i = gl_NormalMatrix * normal;
 }