server: Subtract the current update region from the valid rect when a window client area is changed.

For cube maps, texture->target is GL_TEXTURE_CUBE_MAP and would be used for
calls that apply to the entire texture, like glTexParameter().
surface->texture_target refers to individual faces like
GL_TEXTURE_CUBE_MAP_POSITIVE_X, etc., and would be used for calls like
glTexImage2D(). What makes this a bit ugly is the fact that surfaces don't
always have an associated texture.