Use search path for simple texture expressions

The path to the texture can be in any subdirectory of the root path
or within any subdirectory of the search path. The relative path to
the texture will be discovered automatically.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
There's no functionality in boost filesystem to transform an absolute path to a relative path, 
relative to some other path.
We'd need to write our own. But I think this is mostly an appleseed.studio need.