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>
Merge pull request #512 from marius-avram/texture_search_paths

Use search path for simple texture expressions
OSL improvements.
Relative texture paths in expression fixes

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #515 from marius-avram/relative_paths_fixes

Relative texture paths in expression fixes
Fixed PR comments.
Force travis build.
Reverted changes in test_vector.cpp
Removed microfacet BTDF test scene.
Small fix
Merge pull request #513 from est77/OSL

OSL improvements.
Fixed PR comment.
Updated DisneyBRDF, refactored and fixed bugs in diffuse component.
Merge branch 'master' of https://github.com/est77/appleseed
Merge pull request #516 from est77/master

DisneyBRDF updates #1
Fixed bug