Fix enum string generation

Should be "ENUM_NAME" "enum_nick" not "enum_nick" "enum_nick".
Clean up get_bounding_box for composer/composer3

It is safe to write to one of the inputs of gegl_rectangle_bounding_box
so the deep if chain is unnecessary.
texturize-canvas: Add OpenCL implementation
texturize-canvas: Do not repeat parameter derivation.