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>
the check for simple texture lookups should be here.
you need to check for exactly texture("any_filename", $u, $v) taking into account whitespace differences.
for example, tokenize and match tokens. texture( "filename", $v, cos($u)) is not a simple texture lookup for example.