Prevent changing some built-ins in spatial shaders

[Chaosus]

CUSTOMX are meaningless to be used as output because in shader it's defined like:
layout(location = 6) in vec4 custom0_attrib;
And canvas_item shader already has the same built-in, and it's defined as constt.

Also, BONE_WEIGHT and BONE_INDICES are affected by the same problem.

NODE_POSITION_WORLD corresponds to MODEL_MATRIX which changing is forbidden at previous lines.

CAMERA_POSITION_WORLD corresponds to INV_VIEW_MATRIX which changing is forbidden at previous lines.

CAMERA_DIRECTION_WORLD corresponds to VIEW_MATRIX which changing is forbidden at previous lines.

CAMERA_VISIBLE_LAYERS IDK about this one, but logically it should not be changed in shader.

NODE_POSITION_VIEW translated to (scene_data.view_matrix * model_matrix)[3].xyz and assigned to this one lead to shader crash.

[clayjohn]

I agree. These should be enforced as consts from the shader language side so we can provide nice errors when they are written to.

Right now users get faced with our internal shader compiler error which is not friendly and is super confusing for them

[akien-mga]

Thanks!