Custom Texture expression function

The call to the functions looks something like this:
texture("path/to/texture.png", $u, $v) where $u and $v
can be replaced by any other variable.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #500 from marius-avram/texture_expression3

Custom Texture expression function
Clear, Load, Save expression editor fix

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Split OSL reflection and (future) refraction microfacet closures.
Merge pull request #502 from est77/est

Split OSL reflection and (future) refraction microfacet closures.
Merge pull request #501 from marius-avram/save_load_fix

Clear, Load, Save expression editor fix
Texture expression fix

Fixed a few of the issues mentioned by Esteban.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Moved prep_texture_expr inline

plus fixed bug at line 373

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #503 from marius-avram/texture_expression_fix

Texture expression fix