Add instanced attributes flags to GLSL prefix

[yahel-ck]

Adds #define attrib_instancedFlag for each instanced attribute, mostly to allow declaring an attribute instead of a uniform when the specific attribute is instanced (like I did with u_worlTransform for example).

I tested it with this demo that implements instancing u_worldTransform: https://github.com/yahel-ck/libgdx_instancing_demo The demo is using gdx-gltf to render, but it uses the same underlying LibGDX classes. I tested it on VMs of iOS and Android, and on a physical Android device (Samsung S21).

It wouldn't take much work to implement the demo solely with LibGDX so let me know if you think it's necessary, or if there are any other tests I should make. Thanks :)

[yahel-ck]

Don't know why this says I request another review. No review is needed ATM, I'll unmark this as draft after addressing the requested changes in the depth shader