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>
Oh my bad, sorry. May I suggest to move this functionality to `foundation::SearchPaths`? Maybe as an `unqualify()` method?