Move uniform variables up in the gles3/scene.glsl template so that they are available within the #GLOBALS scope

[alessand10]

This PR should:
Resolve #100080

As well as prevent future issues in which the following uniform variables are needed (yet not defined) within '#GLOBALS'.

The following 3 uniform variables were previously defined after '#GLOBALS' within the fragment shader of 'drivers/gles3/shaders/scene.glsl':

uniform highp mat4 world_transform;
uniform highp uint instance_offset; 
uniform highp uint model_flags;

But there may be instances in which these variables are needed within '#GLOBALS'. To keep consistency with the vertex shader in the same file, I have moved these definitions to just below the

#ifdef USE_MULTIVIEW
...
#endif

Preprocessor directive.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉