MDL: Add support for geompropvalue and scene color data

[bernardkwok]

Currently both of these are not implemented.

1. For geompropvalue the following can be used:

data_isvalid(geomprop_name);

with lookup using these variatns (taken from MDL 1.6 specification)

The standard scene data lookup functions exists in two versions, one for varying data and one for uniform
data.

ltype data_lookup_ltype(
uniform string name,
ltype default_value = ltype()
) varying

Returns the scene data of the give name at the current shade point (for geometry scene data) or the
global scene data. The argument to the name parameter must be a literal string value.
A lookup on an unknown scene data name or one
with incompatible type returns the default_value.

uniform ltype data_lookup_uniform_ltype(
uniform string name,
uniform ltype default_value = ltype()
) uniform

Returns the scene data of the give name at the current shade point (for geometry scene data) or the
global scene data. The argument to the name parameter must be a literal string value.
A lookup on an unknown scene data name or one
with incompatible type or varying type returns the
default_value.

2. Unknown what to do with color, though could be treated the same as geompropvalue with a number which an application can derive the final lookup name.