Misleading naming in shader

[outobugi]

Description

There is some confusing naming going on in the shader. Partly my fault.

• get_region_uv() -> returns texture coordinates, not UV
• uv_center in get_material() is not related to material UV and is texture coordinates referring to domain's/control map pixel's center position

Rename get_region_uv to get_region_coords and uv_center to domain_center or cmap_pixel_center for absolute clarity.