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>
Would [`foundation::split_paths()`](https://github.com/appleseedhq/appleseed/blob/master/src/appleseed/foundation/platform/path.cpp#L121) fit the bill? [Here](https://github.com/appleseedhq/appleseed/blob/master/src/appleseed/renderer/modeling/project/projectfilewriter.cpp#L219) is an example call: we are given an absolute path to an asset (say, a texture), and we want to compute a path relative to the location of the project file.