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